Avatar: The Last Airbender/WMG/Bending

All non-benders aren't actually non-benders.
The reason there are non-benders at all is because each bending culture only practices their native element. Non-water benders born into the water tribe, for example, are actually earth, air, or fire benders. They'll just never open their mind to what they're capable of.
 * Jossed. It has been stated that what element a person bends is based on the Nation of their birth. For example, someone born into the Earth Kingdom can only be an Earthbender. Weather or not a person is a bender depends on how spiritual that individual is by nature. This can be seen quite plainly with Sokka and Katara. Katara, who is a Waterbender, is considerably more spiritual then Sokka, readily believing in things like reincarnation and fortune-telling. Sokka, a none-bender, has trouble accepting things as basic as karma.
 * It was also stated that Sokka had some waterbending potential. Perhaps some non-benders aren't truly non-benders, but just people that let their personalities or beliefs get in the way of developing their bending potential.
 * Very often non-benders have traits of other nations. For example Jet has traits of the fire nation and the Earth Kingdom. Suki has traits of the Earth Kingdom and the Water Tribe. Ty Lee has traits of the Earth Kingdom, the Air Nomads, and the Fire Nation.

Every Air Nomad is an Airbender.
We don't see any Air Nomad who doesn't know Airbending but the real evidence comes from the temples and the genocide. The temples are so high up that the only way to get there is flying with a glider or with a Flying Bison (until airships came around anyway) and only an Airbender could use a glider. Also the temples have been shown to have doors that can only be opened by Airbending which would really hamper movement for non benders and so it wouldn't make sense to have doors like that unless everyone could open them. Lastly, all that Sozin had to do to break the Avatar cycle was kill all the Airbenders and the Flying Bison because then even if a new Airbender was born he wouldn't know how to Airbend without learning from a bender or a Bison however, Sozin killed all the Air Nomads which suggests that they were all a threat because they could all Airbend.
 * Confirmed.

Gyatso sacrificed himself in a true Crowning Moment of Awesome by using a deadly Airbending technique.
Namely, oxygen removal. He lured all those Firebenders to the room where he was found, then used Airbending to remove all the air from the room, depriving the Firebenders of the oxygen needed to bend and depriving them (and himself) of oxygen needed to breathe. This would explain beautifully why he died looking so chill.

The Stronger the Bending the less Benders there are.
Look at the facts. The Earth Kingdom has the most benders and is losing to the Fire Nation which has the second most amount of benders. The Fire Nation hasn't been able to overcome the Water Tribe because the Waterbenders are stronger than Firebenders and the Southern Tribe has only lost its benders because the Fire Nation used its superior numbers to slowly whittle away at the Waterbenders and even then it took almost a hundred years to take out all the Southern Waterbenders but Katara. Lastly, Airbending was so powerful that even though the Nomads only had 4 temples, the Fire Nation needed to launch a simultaneous all out assault on every temple while having their power increased exponentially by the power of the comet in order to defeat the monks. This would also help maintain the balance that is mentioned in Avatar. Air has lowest numbers but strongest bending. Water has Second strongest bending and second lowest numbers. Fire has second most numbers and second lowest bending. And Earthbenders have most numbers but least powerful bending.
 * I think the deal is, the more people there are, the lower the proportion of benders to non-benders there are. And I doubt the Earth Benders are inherently weaker than other Benders. Furthermore, the reason the Water Tribe was able to repel the Fire Nation was because they have incredibly hard to assail terrain and a much smaller area to defend. But the Fire Nation almost took the Northern Water Tribe. Without Aang's appearance, they probably could have sieged and taken it.
 * Conservation of Ninjitsu anyone?
 * This could also be tied to Wu Xing (or just basic elemental theory). Though Wu Xing has five elements instead of the four in Avatar, Wood (could be read as Earth) feeds Fire, thus Earthbending isn't as powerful against Firebending because fire burns earth. Likewise, Water quenches Fire in Wu Xing, which is why it's harder for the Fire Nation to overcome the Water Tribes.

Why is this true? It's because...

The power of your bending is a function of the number of times you've reincarnated.
Population sizes change over time, so the pool of souls can't be constant -- some people must be getting born with brand-new souls when populations increase in size, and some souls must have to hang around in limbo for a while when populations decrease in size. If you suppose that older souls generally get preference over newer ones when there aren't enough bodies to go around, and that souls other than the Avatar either prefer to stick with their own nation or have no other choice, you get precisely the behavior described in the previous WMG. The Air Nomads were all benders because there weren't very many of them, so there wasn't much room for non-bender souls to reincarnate, while earthbenders and firebenders are relatively rare and weak because their recent population growth has led to the creation of lots of new earth- and fire-oriented souls that haven't had time to figure out bending yet.

The Avatar always gets bumped to the front of the line, which is why Aang is better at bending than anyone else. Other benders who are said to be anomalously strong (e.g., Katara, Toph, Azula, and Bumi) also have souls that, whether through luck or fate, have been reincarnated a lot.

Some notes:
 * This is perfectly aligned with the Word of God (quoted elsewhere on the page) that bending is a function of spirituality rather than genetics -- in fact it's probably the most literal way you could possibly interpret that statement.
 * It's also a plausible extension of the way the Avatar works. Avatars learn from their past lives, and draw on their power more directly when entering the Avatar state. Presumably everyone else is doing it too, just more subtly.
 * This means that at least one of two things is true about all the missing airbending souls:
 * They reincarnated into other nations, explaining the presence of people who seem air-oriented (like Ty Lee and Téo) in the Earth Kingdom and Fire Nation, but since they're born into a land that doesn't match their soul's bending style they can't actually airbend. (On the other hand, if the same thing happens to that soul often enough, one of their future lives might start being able to airbend and earthbend or firebend...)
 * They haven't reincarnated at all yet. All of Aang's descendants, for a long time, are going to be airbenders, until the backlog is used up and new Air Nomad souls can start being created.

The rarity and potency of Bending seems to relate the bender's sex.
In the show, it may be apparent that male benders are more common. This may be that male benders are more encouraged to practice their skills, as seen by the Northern Water Tribe. However, it can also be observed that female benders may have much more capacity for bending. For example, most of the on-screen Earthbenders are male, yet Toph is an Earthbending prodigy. Given though, she learned much of what she knows from the Badger Moles, but much of her skills were self taught, and as far as it is known, she is the only Metalbender in existence. Furthermore, female waterbenders seem predominantly capable of healing over male waterbenders. Katara learned how to heal by accident. In addition, she had only been practicing self-taught waterbending for a couple of years, and yet she was able to hold her own against a waterbending master for quite some time before eventually being bested. In the "Puppet Master" episode, it was revealed that exceptionally talented waterbenders are capable of bending blood in the living body, essentially being capable of controlling people against their will. Hama and Katara were the only observed benders capable of doing this, and Hama learned it on her own. The only other bender that was seen doing something similar was Huu, who bent the water within plant matter.
 * First of all, Toph (as you said) Was taught by Badger-Moles, the original Earthbenders, and therefore should simply be a better Earthbender by learning from them. Two, Toph is blind. Think about it, it is this very "disability" that made her such a good Earthbender, and is likely the only way she could learn Metalbending. It is very likely that any blind Earthbender would be about as proficient as Toph, but only if they were ALSO trained by Badgermoles, which is highly unlikely. I really don't think that A bender's power level is determined by their sex. A male bender and a female bender both have the same capacity to be a great bender at birth, but any number of factors can change how they develop and how strong thay are in the end.
 * Finally, the most powerful firebender in the series (based on their level of training) was Azula. Azula was considered by everybody to be a firebending prodigy, and was probably one of the only firebenders besides Ozai who was capable of bending lightning. Given that Ozai was the fire lord, and was probably much more trained than Azula, it seems likely that Azula could have easily surpassed him in power by the time she would be his age. The two exceptions are Iroh and Combustion Man. Iroh got his incredible firebending skills from decades of experience, along with the fact that he learned techniques from the only remaining dragons in the world. Combustion Man's skills were based upon unusual circumtances, and according to Word of God, he could barely control his own powers. The reason he was missing his arm and leg was due to severely harming himself by his own bending, not to mention that even the slightest damage to his firebending source caused his abilities to severely backfire.
 * We don't know how strong either Iroh or Ozai were when they were as old as Azula.
 * Aang is the most powerful bender in the series, but only because he is the Avatar. And in his past lives, Avatar Roku's most powerful onscreen bending is when he bends all four elements simultaneously in "The Avatar and the Firelord". Avatar Kyoshi's most powerful bending act is creating an entire island.
 * Kyoshi did not create the island. She separated the landmass, and she must have used both Earthbending and Waterbending to do it, and an act of such power must have almost killed her. Also remember the States Roku and Kyoshi must have been in during there respective acts of bending. Kyoshi looked to be in her twenties, at the height of her bending prowess. Roku on the other hand, had just been awoken by a volcano was at least in his sixties, was breathing in toxic gases, was worried about both his village and his wife, and was FIGHTING A VOLCANO. Say what you will about Kyoshi's level of power, but her bending target was not killing her while she worked.
 * In "The Siege of the North Part 1", the all male Waterbenders of the Northern Tribe lift a Fire Navy ship out of the water using five or six boatfuls of people. In Hama's flashback in "The Puppetmaster", it takes eight Waterbenders of both genders to do the exact same thing. Also, Hama, a woman, was the last Waterbender to be captured.
 * It's likely that how strong a bender is is determined by how driven they are. Hama was a VERY driven individual.
 * In the Southern Water Tribe most of the benders seem to be female. Hama, Katara, Korra (the next Avatar). The Northern Water Tribe has male benders and female healers. The Swamp benders are all male.

In the ancient times, Energybending was known as Ninjutsu.
One of the Wild Mass Guesses says that Avatar world is the world of Naruto in the distant past. But with the introduction of Energybending, it starts to look a lot like other way around. "The energy within ourselves" the lion turtle talked about was obviously Chakra, and people could use it to a lot of other things than taking away each other's powers. Aang, as pretty much the first Energybender in thousands of years, is thus the first ninja for a long time.
 * And therefore, by Conservation of Ninjutsu...

The Giant Lion Turtles are the precursors to the Avatars
Just as the Moon was the first Waterbender, the badger-moles are the original Earthbenders, the are the original Firebenders, and the flying bison are the first Airbenders, the Giant Lion Turtle seems to be one of the original  In addition, there's his little conversation with Aang: "Giant Lion Turtle:"

Y'hear that?
 * This editor likes to take this to the next logical step and think the Lion Turtle tried to stop the war during the period Aang was in the iceberg. Sure, there are a lot of holes in that, but still...
 * Avatar Yangchen once said that the Avatar has to be human, so it can symphasize with the people that he or she would protect. The Lion Turtles, obviously being non-human, were probably replaced by the Avatars for this reason.
 * Avatar Yangchen once said that the Avatar has to be human, so it can symphasize with the people that he or she would protect. The Lion Turtles, obviously being non-human, were probably replaced by the Avatars for this reason.

Bending strength decreases with age.
There's a pattern which is pervasive: Toph can defeat a small army of other Earthbenders, Katara was able to stand up against Pakku even before she was fully trained, and Zuko clearly seems more powerful than "master" Admiral Zhao (though less powerful than his younger sister... in fact, the only people who have ever beaten Zuko in a fight are younger than him). By contrast, "powerful" old benders we've seen tend to rely upon finesse: no matter how beneficial it would be, Iroh never uses much fire, his ability is demonstrated solely through having more delicate manipulation than other firebenders, just as Pakku has never bent more water than Katara is able to - he's just faster at it. The only really legitimately powerful old bender (aside from the past Avatars) is King Bumi, and he appears to have stayed young in a number of ways. Iroh seems noticeably unwilling to fight for more than a few seconds at a time. Given the physical effort shown, it's fair to say that bending is a demanding art and that your physical and mental health is just as important as your skill. Even Roku looked noticeably tired and weak in his last battle.
 * Evidence against: Zhao's problem is exactly as his master noted -- he's an impatient, pigheaded idiot who relied on brute strength over actual skill. Zuko was simply better, no more and no less. Ozai (who pretty much proved himself Badass with that dual lightning bolt attack) and Jeong Jeong also seem quite powerful.
 * I will note though that likewise, neither have been seen fighting for more than a few seconds at a time. Ozai is astonishingly well preserved (He looks half his age at least) so I would bet on him still being a fearsome fighter, Jeong Jeong I would expect to rely on quick application of overwhelming skill like Iroh. This is of course all generalisations; there are plenty of old men out there who could beat the crap out of your average 20 year old in terms of raw stamina (Iroh's main problem was that, no offence, he was unfit and overweight.)
 * Not any more.
 * Also, Katara and Toph are pretty much evenly matched, as seen in "The Runaway", despite the fact that Toph learned from the original masters themselves, while Katara hadn't even started training until a few months ago.
 * Bending does seem connected to physical fitness and strength, but what older Benders lack in that they can make up with skill. Naturally, ageing Benders who keep themselves fit and practice their powers can keep up with younger ones, while even young benders are little threat until properly trained. Since Bending seems to be connected to physical motions, skill and talent can be more important than fitness.
 * May I remind you that Bumi, who is an unfrozen 112 years old, can bend with his face.

Alternately, bending strength increases each generation.
As the world approaches the spiritual singularity, benders get more and more powerful. Iroh is much more powerful than he would have been at Zuko's age, as well as more finesse and skills that could turn a fight in his favor, he's just not as strong as Zuko is at Zuko's age. Zuko learned to lightningbend in only a few months, Azula mastered it at the age of sixteen, Katara showed up the waterbender to train from so hard he broke his "no teach girls" rule and figured out bloodbending on the fly, Toph bent metal (the finesse from spending years seeing with earthbending and learning from the badgermoles, and the power from being in this generation). The only two people we see from previous generations to have cool powers are, in decreasing order of age, King Bumi (whose only claim to earthbending fame was bending with his face, and he's the freaking Earth King), Iroh (with his firebreath tea and more importantly his lightningbending, which took years to master after figuring out the principles of it amd is, if I recall correctly, under the category of "ancient arts"), and Combustion Man (who... Well, never actually does master the ability to shoot firelazers from his face, but he does manage to do so at all). If Aang had faced up to his duty and learned Earth and Waterbending when he was supposed to, a hundred years ago, he likely could have flattened the entire fire nation (at least without their more-advanced technology) without even resorting to the Avatar Form.

Everyone can bend something.
Nobody can bend two elements except the Avatar. Plus, in order to bend, people need to know how. Before the earthbenders knew about the badgermoles, they didn't know how to bend. This can be applied to the bending of elements not yet known. Since element teachers do not exist or are unwilling to teach, there are no human benders of that type.
 * It has been hinted at several times that bending is something in your genes (or something.) So the people who first learned could have already had the potential.
 * Except that it's not.

The Four Elements thing is a lot less necessary to the world than implied early on.
The beginning of the series pretty rigidly defined bending as the four traditional western elements, but repeatedly blurred the line with lighting, sand, wood, metal, and soul manipulation revealed as time went on. As bending is a combination of religion/martial arts, any one of these sub or splinter disciplines could split off at any time and branch out into it's own culture. The Avatar world 200 years from the ending of the series might have a vastly different elemental, political, and religious structure than the one we were shown.
 * Alternatively, the destruction of the Airbenders and the absence of the Avatar for 100 years has somehow damaged the formerly rigid order of the four elements and thereby opened up the space for the rise of new types of bending. So, the definition of the elements was rigid but is no longer. The people, however, have not yet fully realized the reach and consequence of this change. Remember that the story of the series is more or less narrated from Katara's point of view, and her education before the beginning of the series' events was still based on the old traditions and thought patterns.

The 'Secondary' Airbending Technique is Climate Control
Every bending discipline has one or two secondary techniques, which require special training: Firebending has lightning, Waterbenders can Bloodbend or Plantbend, and "the most powerful Earthbender in the world" can bend metal. We were never shown a proper secondary technique for Airbending - but what is the first thing that Sokka says to Aang when they're at the South Pole? "You aren't cold/frozen!" He could be referring to the fact that Aang just climbed out of an iceberg all hunky-dory, but Aang is still perfectly comfortable despite wearing naught but Air Nomad clothes. Solution: Aang, a master Airbender, can control the temperature around him, without effort. Think about it: Has Aang ever complained about being too hot, or too cold?
 * Alternately, the secondary Airbending technique involves the manipulation of sonic waves. By controlling sound itself by merely moving his hands, Aang will one day be able to unlock the ultimate Airbending move:.
 * Made of win. Fanfic plz nao?
 * Temperature control, or at least heat control, was shown to be a Firebender technique. Iroh brewed tea in his hand without an open flame and Zuko (despite visible fire escaping from his mouth) was capable of keeping homeostasis in extremely cold temperatures.
 * Waterbenders can also control temperature in their ability to freeze, and thaw, not only liquid water, but to form ice out of water vapor in the air. This is kind of only control over their specific element and its form, but that would pretty much be what the Airbenders do.
 * In keeping with the parallel with Bloodbending and Lightningbending, a possible secondary/advanced Airbending technique would be "Breathbending", which in theory should be incredibly powerful and lethal. Given the prevailing culture of the Air Nomads, that knowledge would probably be strictly controlled, and might not have been taught to a 12-year old child, even if that child is the Avatar (they would have wanted to wait for him to get older first). In which case, the secret died with the other Airbenders.
 * Additionally, the lethality of their special technique may be the reason all Airbenders are raised by monks in temples and taught pacifistic ways. Otherwise they could be tempted to use that power to kill whenever they feel like it. Only monks old and responsible enough are told of the technique, and it serves as a last line of defense. It would explain 1) why a special Airbending technique is never shown, 2) why every Airbender and only Airbenders live in the temples, and 3) how Gyatso managed to kill all those super-comet-powered Firebenders. Also, the Avatar is probably the one person they wouldn't have told about it; he/she (depending on the incarnation) can control the other three elements, and so has no need of such a powerful Airbending technique. Normal Airbenders would need it more, since air probably can't do as much damage as the other three elements.
 * This could also be part of the reason Sozin targeting the Airbenders first for extermination. Breathbending could in theory completely shut down all forms of firebending, making the Airbenders the biggest threat to his plans, new Avatar reincarnation or not. Taking them out with a surprise attack first with the comet power-up would be the only way for him to beat them.
 * If you think about it, it's a natural extension of airbending. All Aang has to do is excite the air molecules surrounding him to create a comfortable temperature. When it's hot, he just creates a personal breeze.

Waterbenders lose their power during a Lunar Eclipse.
It makes sense, since Waterbending is the opposite element to Firebending. On that note, since Waterbenders are strongest during the full moon, they would logically be weakest during the new moon.
 * I think Katara expressly says this at one point.
 * Katara does say this while they are in Library trying to find the Day of Black Sun.

The four elements are thematically linked to different subjects.

 * Water: Close personal relationships. (Water Tribe)
 * Earth: The individual. (Earth Kingdom)
 * Fire: Culture. (Fire Nation)
 * Air: The world. (Air Nomads)
 * Their order (Air, Water, Earth, Fire, Air) reflects a person's evolution: first only physical sensation, then a recognition of other people, then a sense of self, then indoctrination into a culture, then encountering the realities beyond that culture.

The real reason the Air Nomads were attacked first is because air is the strongest element
Point one, Aang kicks the crap out of hordes of firebenders, and never got into a serious fight with Zuko, he always kicked his ass, and he had was four years younger. Only Azula proved to be a challenge for him, and she, the greatest living firebender of her age, needed help to fight Aang. Point two, the only other example of airbending was done by Gyatso and he was surrounded by a dozen corpses of firebenders at their most powerful. The airbenders were attacked because they would have destroyed the Fire nation.
 * Yeah that makes sense to this troper as well. Aang devastates every firebender he sees (granted he is the Avatar but he is still very young and relatively untrained compared to the firebending army). Additionally airbending has the least amount of restrictions on it as a bending art. It can be used with minimal movements (a sneeze is sufficient enough to knock out two guards in the pilot), it has plenty of source material to be bent and can't be weakened by natural phenomena like eclipses, grants the user impressive speed (Aang can generate hurricanes by running), and is substantially long range. With all these strengths, the Fire Nation didn't stand a chance unless they used a comet to power up and even then it took the ENTIRE army to take over FOUR temples filled with PACIFISTIC MONKS.
 * By the time the series starts Aang is a master Airbender, most Fire Nation Soldiers are not master benders. Heck Zuko, the guy that could defeat Elite Mooks of the fire nation, wasn't a master firebender until season 3. Pitting any number of soldiers against Aang, with him just being a master Airbender, is simply foolish without some kind of overwhelming advantage, adding in the fact that he's the Avatar, and you have to have master benders with an overwhelming advantage, or failure is certain. I doubt that Air is the strongest element.
 * Gyatso was a master Airbender as well, but you should note the pile of corpses that were in the room with him. Corpses that were, when alive, powered by Sozin's comet. Or, if the comet power had run out by then, it would mean that Gyatso had killed them all in one Last Dance- esque final blow
 * Airbending is the most dangerous form to Firebenders, because fire can't exist without air. Master Airbenders can snuff out fire.
 * Here's a basic point: Airbending is primarily a defensive style. (think of the elements as the classes in an RPG: water=healer, fire=DPS, earth=tank, Air=defense) this not only points out that Gyatso was easily one of the biggest, non-avatar badasses in the series (killing a very large amount of firebenders on their strongest time with DEFENSIVE bending) but it points out that Airbending is so ridiculously powerful, that even as a defensive style it can outmatch every other element. what would happen if someone used it offensively? or did something like bloodbending and uses airbending to suck the air out someone's lungs/compress the air around them until they're crushed.

Energybending is the source of all the "special" Avatar powers, beyond the four elements.
This works better if the unofficial name you use for it is "Spiritbending" instead of "Energybending". In addition to bending the four elements, he can also manipulate spirit. Going from this, let's look at the non-bending powers the Avatar has. What Aang did to Ozai was simply him using Spiritbending on another person, rather than on himself all the time. The Lion Turtle even says that his "spirit must be unbendable". This makes spiritbending easier to swallow, and  explains the other Avatar powers, at the same time.
 * Astral projection into the Spirit World: This is the Avatar bending their own spirit out of their body, where it can enter the spirit realm or wander the world invisibly.
 * The Avatar State: This one is real easy. It's already been described as the Avatar using the knowledge and power of all the previous Avatars at once; pulling all their spirits into one body. This explains, of course, why dying in the State would end the cycle of reincarnation; the entire Spirit of The World that is the Avatar is in one place at one time.
 * What he did with the Ocean Spirit: That was him merging the Ocean Spirit with his own; gaining its powers, much like what he does with the other Avatars in the Avatar state.

By our time, the only bending styles left will be those of the last three chakras.
A new age is coming with the finale, right? Anyone else find it strange that four chakras correspond to bending elements while others arbitrarily don't? That leaves us with Soundbending (or Aetherbending, if you'd rather go with The Other Wiki's element listing as opposed to the ones Pathik states), Lightbending, and Thoughtbending. These sound like the ways you'd influence someone with peaceful speaking and compromise. These are the only ways left to end conflict, as opposed to throwing earth/water/fire/air at the other side.
 * Except Lightbending, which sounds like threatening the other side with lasers.
 * Peaceful? They sound more like "sonic blasts", "lasers and illusions", and "Mind Rape", respectively, to me. Perhaps they are lost arts, or, especially in the case of thoughtbending, deliberately suspressed.

Firebending is an offshoot of Energybending.
It would actually make sense. First, look at energybending- in the series, it is seen as manipulating the spiritual energy (chi) inside a person. Now look at the four elements. Water, earth, and air are all aspects that exist naturally in the world. Fire, however, is not. It's entirely possible that during whatever event that cause energybending to split into the four styles, firebending continued to focus on energy- however, rather than focusing on spiritual energy within the body, it focuses on natural energy outside of it. After all, fire is simply a chemical reaction that releases lots of energy- Aang even says that fire is energy. So, it's entirely possible that firebending is a repurposed form of energybending.
 * In this train of thought, lightning could be seen as being even closer to energybending- creating it involves manipulating energy inside one's body. However, it isn't a perfect form of energybending- while there is the seperation that creates lightning, it lacks the fine manipulation seen in other bending styles. In theory, it's entirely possible that energybending, if utilized in a certain matter, would result in the use of lightning in a way that would be similar to the other bending styles- in short, lightningbending.
 * Taking that idea even further, Bending is turning the body into a humanoid electromagnet, controlled by complex neurology that may as well justify its telekinetic properties.
 * Bending not only 'resonates' with the movements and states of certain 'elements', but also other bending practitioners (at least in childhood, at a certain point, the 'frequency' of resonance is hardwired), thus why learning from the Source benders gives the observer almost instant mastery of that element. That is why Oma and Shu could learn instantly from the badgermoles, why all Air Nomads( who lived with their Source-Benders were Airbenders), why Tui and La took mortal form, and why Ran and Shao could effectively restore Zuko's bending in one burst of flame. Same goes for the Lion Turtle teaching Aang.
 * Also, nerve-impulses, which conduct the electricity needed for bending, can be interrupted by the likes of Ty Lee to hinder bending or jarred to wake up buried powers e.g. a hard poke to Aang's back restored his Avatar State.
 * This also traces back to real life manipulation of 'chi' being a reflection/medium of manipulation of nerves and reflexes.

Koi fish are the water bender's animal equivalent.
It'm not all that wild, but Tui and La are the moon and ocean, and if the moon is the original source of Waterbending. Earth has badgermoles, Fire had dragons, Air has flying bison, and Water has koi fish.
 * In that case, they'd have to be magikarp leveled.

Nation dictates what kind of bending one can do, spirit determines whether they can bend or not.
If you are born into the Fire Nation, you have the potential to bend fire. However, not everyone can bend. Anyone who has the "spirit" of a bender can bend. Or, whoever has enough hope can bend. That's why Katara can bend and not Sokka. He was really skeptical about the whole "spirit world"/bending thing. Katara on the other hand had a lot of hope and believed in all that spirit world magic.

Also, this is what makes benders good or not. Since Katara had so much hope and spirit, she became an amazing bender very quickly. Toph, who had a strong passion for bending, was able to learn bending easily from the badger moles. Zuko wasn't the greatest bender for the longest time because he didn't believe in himself or have much hope. However, he still was a bender because he was once a hopeful, spirited kid. This is why all Air Nomads were benders; they had so much spirit.

Finally, the Avatar was the most spirited, gifted, passionate and hopeful Bender born when the previous Avatar died.

Although bending is not genetic, people believe it is.
Their conviction that bending is inherited is what causes only people with Air Nomad background to be able to Airbend, and so on for the other elements.
 * It's stated in the supplimentary material that the Airbenders were the most spiritual of all of the nations, which allowed them all to become airbenders.

Bloodbending is not evil.
Just because one person abuses something doesn't mean it's inherently evil. Ignoring for a moment that bloodbending can only be used during a full moon, imagin how it could affect the medical community. Bloodbending could be used to stop people in accidents from bleeding to death either internally or externally, would probably be a big help in operations and births, and at least a few health problems can be remedied by simply thinning the blood, like using aspirin during a heart attack, but faster. Katara couldn't imagine a way to use it without hurting people after seeing how Hama used it, and most other people didn't know about it.
 * Bloodbending the way it was used in the show, however, probably is. Controlling people like puppets tends to be a cultural taboo. It was, also, used rather unimpressively in the show. If all you can do when you're bending the water inside a person is use them like a puppet you're not very imaginative. You'd have better results if you just ripped the water right out of them. Especially true for Hana who was actively hurting people with it.
 * Manifesting the Chunky Salsa Rule on people would be evil, but Hama is good! She'll tell you so herself.
 * It's less likely that Hama is unimaginative than that she simply wants to use Bloodbending the way she does. When Katara mentions Hue manipulating plants by bending the water inside them, after all, Hama tells her about pulling the water from plants for raw use; she's surely aware of the potential to bend the water out of someone's body.
 * I don't think they can show a person exploding on a kid's show.
 * Explain Combustion Man, then.
 * Okay, having someone explode in a shower of blood would not be appropriate for a kid's show.
 * How about just simply stopping the blood from moving? 5 minute death, guaranteed. Or manipulating the flow of fluid in their brains to change the ionic concentrations around their neurons? Mind control. Pushing the blood out of their head and into their extremities? Non-lethal knockout (if stopped soon enough). Accelerate blood flow through the kidneys and/or liver? Cures most poisons. (Oh, and removing the water entirely won't cause Chunky Salsa Rule, as the solid parts of the blood, ie everything that would be red and chunky in it, would be left behind. It would instead be instant mummification. Not any better, of course.)

Sozin's Comet powering up Firebending makes perfect sense.
Hear me out: when the comet finally arrives, it dips into the atmosphere and sets the sky on fire for a while. The effect of this is to produce a gigantic sun that gives Firebenders much more power than the normal sun does. And the reason the atmosphere being set on fire doesn't kill everyone is because of the Spirit World acting like a forcefield and pushing the comet away. That's why a comet that's low enough to get that reaction doesn't just plunge right into the planet like it should.
 * This would probably fit better under Fridge Brilliance rather than WMG.

The animals with the ability to Bend are the result of spirits mating with mortal animals.
We know that the Flying Bison, Dragons, Badger Moles  are the only animals who display any kind of supernatural power. They also appear to be far more intelligent than any other being (if not quite up to human standards). They are probably amongst the most normal looking creatures (considering how the Spirits also tend to resemble normal animals). And there are plenty of Eastern myths about spirit or celestial animals mating with regular animals and creating offspring with supernatural powers. Tui (the moon spirit) was unable to do this, as its spirit form was too large and powerful to mate with mortal animals, and its incarnate form wa ill-suited to creating offspring which would have been effective teachers, so it decided to teach them through the physical moon itself.

Benders in the series are not actually limited to earth, water, fire or air.
Considering the number of times we've seen characters bending something that doesn't exactly follow the elemental chart I think that they are instead bending the states of matter. Waterbenders actually have the capacity to bend liquids, Earthbenders bend solids and Airbenders bend gases. The Firebenders are the only group that don't follow this rule because they bend heat, which also explains why Firebenders don't need to carry candles with them. They simply bend the heat around or inside them.
 * Actually fire is a form a plasma, which is the fourth state of matter. Check The Other Wiki for plasma. As for them being able to bend each others elements (like water benders bending ice) it has to do with the fact that all benders could learn other forms of bending, but don't know it, and cases like this are on the edge of passing from one state of matter to another (or from one style to another).
 * Nice, but waterbenders can bend both ice and steam, which they shouldn't be able to if they bend liquids. Also, the only non-water liquids they're shown bending contain a large percentage of water anyway.
 * Related theory; Benders are not limited to earth, water, fire, or air... They only think they are, and they can actually manipulate other forces of nature. The existence of Oma and Shu, the first Earthbenders, along with other ancient yet unnamed peoples (the first Firebenders who studied dragons, the first Waterbenders who studied the moon, and the first Airbenders who studied skybisons) shows that bending originated from four entirely separate sources. So, if humans managed to develop bending four different times, who's to say that they can't do it five times? Or six? Or an infinite number of times? What's to stop some strange Nature Hero, fully detached from the four nations, from studying a beehive and learning to bend the hivemind? What's to stop someone studying light and shadows from learning Lightbending? Or someone studying physics from learning Gravitybending? My god, the fanfic possibilities are endless.
 * There really is evidence that everyone in that world can bend much more than they think. Guru Patik and the Swamp guy who bends vines both said how everything is connected and even the division of the 4 nations was an illusion. Waterbenders can bend ice, water, and steam, and even change their temperatures, which is a property of firebenders. Firebenders Can bend fire and lightning. Earthbenders can bend earth and metal. I believe that airbenders can bend lightning too. They just need to build energy in the air instead of inside themselves. I think Earthbenders can bend wood. I think both waterbenders and earthbenders can bend lava. If things can stretch that far maybe they can learn one another's bending disciplines.

Bending ability is a mix of personality/spirit and genetics/upbringing.
All people can bend an element. The element you can bend is dependant on personality traits, character, soul, whatever you want to call it. If you are born into the nation of your inate element you have the ability to bend. This could be due to genetics, or to being raised in the correct biome, or to the availability of teachers of that particular element. In this troper's mind Mai is a waterbender, Ty lee is an airbender, Sokka is an earthbender, and ironically Jet is a firebender.

Other Bending Speculation
Here the troper hive mind will think up new bending disciplines, such as the ones talked about by Guru Pathik (sound, light, thought) or other random things that sound cool. Each new discipline would be a nation with its own Fantasy Counterpart Culture and maybe one or two secondary powers for masters. Here we go:
 * Sound - manipulates vibrations through the solid, liquid and gas. Can create complicated music without any instruments that nonbenders would need a whole symphony to replicate. Warrior soundbenders can overwhelm enemy ears with enormous blasts of sound without being harmed themselves, or alternatively they can prevent the enemy from hearing each other or even alter what they hear. Can use soundbending to move completely silently. (Need a culture/secondary power.)
 * Maybe an African-esque culture, considering the focus on drumming and other musical traditions. More powers could include being able to hear long distances or using vibrations can mold materials to different shapes or or create solid shapes out of air by 'tuning' the natural vibrations to make them faster/slower. A secondary power could be sonar, like a bat. The ability to "see" objects that are cloaked or are far away by releasing a wave of sound. Or perhaps the ability to instantly determine a solid object's resonance frequency and literally shake the object apart by generating an intense sound of the same frequency. They would live on the plains, where sound can travel long distances without being interrupted. Sound is the element of communication.
 * Light - can blind people with light, or create darkness so absolute that you can't even see your own hands. Useful in stealth missions. (Culture and secondary powers?)
 * Maybe an indian culture. Secondary powers could be being able to create illusions or pictures by bending the colors of light. Alternately, you could bend the UV rays and give people horrible sunburns that make Zuko's look like a little pink
 * OOH! Even better idea! The light benders are the greek nation - the states of light. They are very diverse, as is light itself being made up of so many colors, and they devote themselves to the arts as the greeks and romans did with their ability to bend color. Light people are democratic and given to many different ways of thinking and ideas all at once. Light is the element of unity.
 * Notably, Sol Invictus became the dominant god in Rome in later stages of the Roman Empire. Maybe Lightland is corrupted, manifesting a nice appearence that hides severe social problems. On the other hand, it could house the closest in Avatarverse to a place where the Inquisition exists.
 * Thought - telepathic communication. Can influence, though not control, other people's thoughts, or just plain talk to them over long distances. They can talk telepathically to nonbenders, though only thoughtbenders can initiate contact. Thoughtbenders are very valuable for every army as messengers and coordinators. They may work as therapists or psychiatrists. A secondary power might be dreambending. They could psych out the enemy with horrible nightmares, or get paid to provide wonderful dreams of your choice. (Culture?)
 * I'm leaning towards some sort of Native American culture.
 * This troper was thinking more of a Russain-communist culture, but one that actually works. See, the thought benders are very dangerous, with the ability to bend people's wills and all. If one were to go rogue, that would mean big trouble for the world. So the thought benders are all constantly kept in check via a hive mind of the entire nation as one. They are philosophers and thinkers, constantly analyzing and speculating the world at large to come up with more knowledge - like an entire nation of WMG. They seem stoic and cold from an outside perspective, but that's only because they experience conflicts and emotion on a deeper level. Thought is the element of control.
 * This society combined with the ability to communicate in groups over vast distances would create a sort of "internet culture", where there are drawbacks to the reduced individuality, but the anonymity and equality makes everyone more at ease and more willing to share their true opinions (though this might not always be a good thing.) A nation like this might be rather isolationist, because anyone who couldn't participate in their giant group debates would be of little interest to them. They could develop a class system, though, where the farmers/laborers have no time to contribute to this nationwide forum.
 * Here's one this Troper used as villains in a tabletop RPG campaign: Shadowbenders. Actually a bit misnamed, as shadows don't really have any substance to bend. What shadowbenders actually bend is Void - sort of the dark side of energy. This element manifests in the world through shadows and darkness, so to an observer, it looks like the bender is grabbing a shadow off the wall and beating you with it. Shadowbenders are capable of devastating physical attacks and are masters of stealth - literally cloaking themselves in shadows. The most powerful ones (who are able to really grasp the Void/Shadow connection) could actually teleport between different shadows.
 * To emphasize their status as outsiders (literally - the Avatar had imprisoned their entire civilization outside of the world thousands of years ago), the Shadow Republic was based mostly on Russia, both pre- and post-communism, rather than an Asian culture. The people of the Shadow Republic were pale skinned and dark haired. Their color motif - unsurprisingly - was black and gray. Shadowbending itself was described as a quick, brutally efficient art, reminiscent of Krav Maga.
 * The Gaang actually meet a Shadowbender in one of the comics. No, seriously.

Anyone can learn all the elements
They just haven't tried, since they think only the Avatar can or it's too hard.
 * This is suggested by some of Guru Pathik's descriptions of bending, and was followed up by Word of God describing the divides as more religious than genetic. (The ability to bend at all seems to be genetic; the elemental locks are religions/social.)
 * If someone were never exposed to the concept of only bending one element, they could potentially to bend all of them!

The Northern Air Temple Refugees will eventually learn to Airbend.
Word of God states that bending is not genetic, but spiritual. Because they are living in a place where the Air Nomads lived and died, their decendents will feel connected with Air Nomad culture and spirituality, and will someday learn to airbend.

Firebenders can't really bend fire.
What they're actually bending is hydrogen and causing combustion with the air. This would explain why in the series, the Firebenders don't need a source of fire. The movie changed this so that they really are bending fire and not hydrogen. We're never told this in the series because "Hydrogenbenders", "Hydrogen Nation" and "Hydrogen Lord" sounds unbelievably lame.
 * Fire is really just ionized gases, and with hydrogen being a gas, we can all safely assume that they are indeed bending "fire".

There are no nonbenders.
This kinda goes in with a few of the above theories. It's not that people can't bend, they just have the wrong temperment/mental setup for the nation they are born in. Because they're raised in places where it is either this or no, they aren't taught the skills, or no one where they are can teach the skills.

Sokka and Ty Lee have skills that are similar in style to an airbender's, but it isn't obvious enough and the belief that only Air Nomads can means that no one teaches them and they don't try to learn.

There's a reason for Earthbenders' strong stance.
When an Earthbender pulls a rock out of the ground, they aren't just lifting it up, because they also have to compensate for the sudden vacuum they've created between the earth and the piece of earth they just yanked.

Hei Bei once taught his own kind of bending to humans
And it was called soundbending. This is of course based purely on the fact that there is a sound chakra in addition to the elemental chakra's (thought and light were also disciplines in this theory) and Hei Bei attacks by yelling.
 * He's not soundbending, he's just FIRIN' HIS LAZOR!!!1!

As firebenders get older, they have to drink a lot of tea to keep from burning up from the inside.
We've seen that Iroh, the oldest of the named firebenders, drinks tea like his life depends on it. We've also seen Jeong Jeong, Zhao, and Ozai drinking tea. Iroh keeps trying to get Zuko to drink tea, not because he's trying to share his passion for the stuff, but because Zuko is so intense that he might burn up sooner if he doesn't drink enough tea. It all makes perfect sense!

Another 'There are no nonbenders' theory: Everyone has the potential to be a bender, but only a small amount of time to first use it.
If a person doesn't learn it from someone or something before a certain point in their development, then they can't learn it later on. This explains why the Air Nomads were all airbenders because they were probably all taught to airbend at birth. The avatar just doesn't have a developmental time limit so s/he can learn all the elements throughout his/her life.
 * That could explain why everyone we've seen in the Fire Nation royal line can bend; power had clearly been very important to them for a long time, so of course they would start training the kids as early as possible to make sure they didn't come out totally worthless (as they would have seen it). They may not have known about the window of time, but they would probably have tried training them because they wanted/expected them to be firebenders, and thus unknowingly ensured that they would.

Lightbending can be achieved by firebenders
Many people before me have commented that this would be a radical totally new way of bending arts, but actually it is a technique that fits perfectly within the range of possibilities of a firebender that works hard enough. Firebenders can pretty much generate lightning, which is considerably different from flaming gas, and they do so by seperating the ying/yang energies sorrounding them. Thus, it makes sense that a person capable of manipulating the atmosphere, heat and energy (making firebending a weird hybrid between air and energybending?) could also control electro-magnetic radiation; its just that nobody actually thought of that, because light doesn't seem like a particular frigthning thing to people unfamiliar with its true potential. Even the Sun Warriors themselves, for all the glorification of fire as a celestial, radiant life force, simply missed that all the light of the Sun could be manipulated into pratical ways. Or maybe they DID knew about the potential, they just didn't thought it was particularly relevant to the state of affairs the world was on (after all, most likely they use light to help grow plants and shit, instead of making laser beams).

Ultimately, however, someone would discover that firebending can extend to the light of the very flames they produce, which could perhaps make them extremely powerful, since both fire and lightning produce light, thus offering potential fuel to lightbending. Most likely this method has limitations; maybe it needs a specially state of mind and spirit, like electricity bending, or maybe it is restricted to daylight hours. However, once learned there will always be one or two users. If the number is that limited such people presumably can be used as people of mass destruction; after all, lightbending isn't really needed when volcanic soil is nearby, but a person shooting lasers would be very usefull to a future mad lord like Ozai...
 * While this seems likely, there seem to be a few flaws with it. For one thing, it'd be incredibly difficult to do, given that it's many steps above Lightning. For another, a Firebender trying to bend light would likely need to actually understand the physics of how light works, something that's probably significantly outside the protosteampunk physics of the fire nation.
 * You do raise an interesting idea though. If the yin/yang thing is how lightning bending works, well, that's rather like the positive/negative charge that builds up with, you know, actual lightning. Which implies that, taken far enough in that direction, a Firebender would be able to bend Energy (Not spiritual energy, actual energy), and possibly even subatomic particles.
 * Now add the Unified field theory and make Avatar a planet in the Marvel Universe. Azula with command over gravity, electromagnetism, and the strong and weak forces, as a Herald of Galactus? Whee!

There is healing potential in every bending discipline.
However, only water was ever associated with healing because of it's more obvious connections with the flow of blood, chi, etc... Bending is the manipulation of different types of energy, and healing is just a matter of using that energy in the right way.
 * I can dig that. Earthbending could be used for stablizing broken/fractured bones, firebending would be helpful for stablizing teperatures (homeostasis) and cauterizing wounds, and airbending sure would make CPR a whole lot easier.
 * Not quite what I was thinking of, but those are very good points. Mostly I got the idea from one of the fics on the recommended page where

Bending could potentially be used to create life.
Scientifically speaking, all life originated from a fluke in time when the hydrogen, oxygen, water, and whatnot were all at just the right levels when lightning (or some other natural energy beam type thing) hit the area and created the first cell. So, if a sufficiently advanced Avatar somehow knew the exact right levels of everything and then created lightning, he or she could potentially create life.

Waterbending is powerful at the full and new moons, and weak at the quarter-moons.
The season 1 finale makes it clear that it's strong during the full moon, but it doesn't actually say that it's weak during the new moon; since waterbenders originally learned from the tide, it'd make more sense for them to be strongest when the tide is highest -- which happens at both the full and the new moons.

Why does Hama only bloodbend at the full moon, and not the new moon? There are several possibilities:
 * New moon waterbending strengthens different techniques than full moon waterbending -- maybe plantbending or healing is easier at the new moon than at the full moon.
 * Hama believes that her bending is strongest at the full moon, so she's never tried bloodbending at the new moon.
 * It's harder to tell when the new moon is than when the full moon is -- you have to look for something that isn't there rather than something that is.

Women are better at Waterbending than men.
Waterbending comes from the moon's pulling of the tides. The moon happens to run on a 28-day cycle, which women can certainly empathize with better than a man can, due to their own monthly *ahem* full moons. Sort of puts Bloodbending in a new light...
 * Well, that certainly makes the traditions of the North Pole sort of dumb.
 * Hardly surprising.
 * The Northern Water Tribe's laws could actually play right into the idea, if we assume that they're a patrical society dating before the first Waterbenders. Once the men realized how much more powerful the women had the potential to be, they relegated them all to the Healing Hut.
 * Alternately, maybe the "men fight, women heal" dictonomy wasn't originally patriarical suppression. Not everyone prizes fighting above healing (Jeong Jeong, anyone?), and given the heavy emphasis on yin and yang plus the fact that warrior tradition seems to be much more an ingrained part of Southern Water Tribe culture, it's plausible that the Northern Tribe originally favored healing. In that case, it wasn't that women were seen as unfit to fight-- it was that men were seen as unfit to heal. Only when the world got thrown out of balance and fighting became such a big issue did the Northern Tribe's values skew the other way.
 * There's also the fact that water is a feminine element.

Those with latent bending potential can utilise it in times of great need
But it won't necessarily work well. Had this one in my head since reading that it's Word of God Sokka had latent waterbending abilities but that his mindset meant they were never realised. We've seen bending happen accidentally (Aang's sneezing) or as a reaction to strong emotion (Katara in episode 1, Avatar State several times) meaning that it is not limited to the forms used in more controlled, conscious bending techniques. Perhaps a person with latent bending potential can instinctively cause bending in the same manner when under threat or the like. But because their powers are so weak it wouldn't happen often or with much/any control over the form the bending took outside of removing the aforementioned threat. An outside influence would also be helpful or necessary - strong sunlight for firebending, nighttime or a full moon for waterbending (in flight for air, underground for earth?) In Sokka's case he really doesn't get the 'magic' business and faces tough situations as a non-bender ("This is called Sokka style, learn it!"), so it'd probably take something pretty dangerous - and a whole lot of water - for his latent ability to manifest itself because it naturally wouldn't occur to him as a solution.

Earthbenders are naturally resistant to blunt trauma.
Earth Rumble contestants don't wear protective gear, and yet none of them seem much the worse for wear the day after the match when they gang up on Toph.

Anyone can bend the elements.
...or, more specifically, anyone can bend the element of the kingdom they belong to (e.g. with the right training, any person in a water tribe could train to bend water, not just the few people who seem to have the skill).

Apparently, all of the Air Nomads knew how to bend air because of something to do with...chakras? (my memory is a little fuzzy on the whole thing) It probably takes a lot of training, but if you try for long enough, you can unlock something inside of you that allows you to bend the element of your people.

Benders are 'powered up' by various forms of radiation.
The theory is, fire benders are greatly effected by solar nuclear radiation. The sheer amount of radiation put out by Sozin's Comet tearing through the atmosphere could certainly boost firebender power levels as well. This might imply that a firebender would actually be strengthened by radioactive fallout, rather than sickened. If the moon is some sort of artificial solar-radiation blocking EM generator, then it makes sense that a solar eclipse would block nearly all Firebender powers.

As for the other elements: waterbending is effected by the force of gravity, that's pretty well established. But what I think actually powers waterbenders is Cosmic radiation. I have a number of reasons for thinking this. First, if we assume that firebenders are powered by Solar radiation, then it is worth noting that Solar radiation pushed back against Cosmic radiation, and fluctuations in the sun's nuclear output changes how much or how little Cosmic radiation reaches us. This gives us a Fire vs. Water scenario. Secondly, Cosmic radiation has in recent studies been linked to cloud formation. As in, clouds might not actually form without Cosmic radiation penetrating our atmosphere. Cosmic radiation is already waterbending! Third, the part of the planet which recieves the most Cosmic radiation is the North and South poles, where there is the smallest amount of solar radiation blocking it (Cosmic radiation is not impeded by our Electro-Magnetic field in the same way Solar Radiation is).

Earthbending might be effected by Electro-Magnetic fields. This may explain why metelbending is so hard: Toph is unknowingly drawing on the metal for power, yet is turning that power right back at the metal she is drawing it from. The lower the concentration of magnetic materials in a given rock, the easier it is to bend it. Note that I am saying earthbenders draw power from metal, not that their powers are magnetic. This may also explain how Toph can 'see' with earthbending: she is sensing not the earth, but all of that various power sources (metal) within the earth. Sand is "foggy" because desert sand is almost pure silocon and very little traces of metal.

I'm not sure what airbending might be powered by. Any guesses?
 * It's pretty specific that Toph actually "sees" using vibrations sensed through the earth. Sand doesn't transmit vibrations well, so she has difficulty "seeing" through it. Although metal concentrations probably do make a difference in ease of earthbending.

Guru Pathnik is the only human Energybender (besides the Avatar).
He uses a sort of telepathy with Appa, and uses Energybending to stay alive for so long. He was abandoned as a young child - whatever nationality he is by birth doesn't matter - in a forest on an island that just so happened to be the Lion Turtle. Subsequently, the Lion Turtle took it upon himself to raise him, and he was raised to be extremely spiritual and to bend the energy within himself.
 * according to another WMG since the avatar is so old that is why he / she is a master of all forms of bending, so could the Guru be as well.

Non-avatar benders can master all elements too, If they're Determinator enough.

 * The creators stated that although bending itself is passed down genetically, which element you bend is based on spirituality. If you had a flexible and open minded but still stubborn person (Air and earth benders are masters at each end, respectively) then they could basically bend whatever the hell they wanted. Granted, it would probably take much longer to master than an avatar would take, and even then stuff like  would be impossible, but Wouldn't it be awesome?
 * Maybe the first avatar started that way.
 * It's actually the other way around. The ability to bend is spiritual; the element bent is genetic. Thus why we have pairs of twins with only one bender between them, benders with non-bending parents, etc.

There weren't any airbenders being born because of the spiritual connection between the Spirit World and the Human World
Okay, I really didn't see if this has already been posted here, but here's my guess: Before the war, there was balance in the world. There were waterbenders in the north/south, earthbenders occupied the main continent, firebenders had their islands and airbenders had their temples. Everything's great, but then the war happened. When it did, all airbenders died, except Aang. We all know that. Let's consider what has happened: The embodiment of that world's god is frozen and in statis for a century. That has got to unbalance things, at least a little. Then we have the airbenders being killed, almost all at the same time. Their homes are burned and everything.

Spirits like that giant... panda... thingie have shown us that messing up with balance makes things go shitty, so what if all these factors had messed up with the world balance, in a way that airbenders weren't able to be born naturally? Word of God stated that genes have nothing to do with bending, so if all airbenders were to be dead, this shouldn't make everyone stop airbending. Maybe what happened was that the world was not ready for more airbenders, because the balance of power had changed.

Firebenders should be a smaller breed. They were confined to an island, and their technology was not so advanced as the Earth Kingdom's earthbending architecture. But when they killed the airbenders, they started to gain power. On the other side of the specter, Water was always confined to places with lots of water, like the poles. But when the firebenders attacked them... see where I'm getting at?

And then there's the earthbenders. They occupy most of the main continent, and Earth is strong to Fire. The strongest earthbenders were born either before the war, when there was peace and balance (Bumi) or after many years of the war, when a new balance was trying to settle (Toph). Those caught up in between (Haru's dad, the other earthbenders at that boat) were "normal", not really with the "gift" these strong earthbenders had. Same thing with Azula or Zuko: She was born when Fire was at its strongest, and Iroh is a hardneded warrior that was there to get stronger as the power of the Fire grew.

So... I had more arguments, but I think I've made my point. There are probably many plotholes in this theory, but at least it's a good guess... right?

Every woman and/or man in the Avatar world can waterbend a little, but it's subconscious, and they use it as means of contraception.
The Avatar world doesn't have the technology to produce modern contraceptives, yet no one in the series has more than two kids. The explanation for this can't be a high rate of child mortality, as dead children/siblings are never mentioned by anyone, and even if mortality was high, some families would still have multiple children. Therefore, the only logical explanation is that in the Avatar world there's a form of contraception that doesn't exist in our world. Since the existence of bending is the biggest difference between the Avatar world and our, it would make sense that this special contraceptive method is based on bending. The answer, then, must be that every woman (possibly every man too) has a very limited waterbending skill, but they are not aware of it. When they are having sex and don't want to have babies, they subconsciously use this waterbending to stop the sperm from entering the ovum.
 * Wouldn't it be more logical for women to just have a longer and/or more stable menstrual period, giving a longer and/or more stable safe zone? The fact that only the avatar can bend more than one element is pretty well-established and sperm/semen doesn't contain significant amounts of water.
 * Longer menstrual period should mean longer ovulation too, so the risk of getting pregnant wouldn't really change. And the "safe days" method is never completely safe, so there should be families with more than two kids. As for the "fact that only the avatar can bend more than one element", that's an established fact only because no one has ever seen anyone bend more than one element... But if the "contraceptive bending" is subconscious and happens on a cellular level, then no one has ever witnessed it. It doesn't even necessarily have to be waterbending, it could just as easily be that every woman (or man) is able to subconsciously use minuscule amounts of firebending to kill the sperm and/or the egg.
 * There was a herb in Ancient Rome that had 100% of chances to make a woman not have babies. They didn't need technology, just basic botanics. The herb doesn't exist, by the way. The romans ate all of them. The only records are from doctors from that time. The point is: There are a ton of other logical explanations for women not having so many children. The waterbending thing looks like it came out of nowhere, and they can bend water, not liquid. There isn't enough water in sperm to bend it.

Bending element is hereditary, but only up to a single generation
The avatar world is very, very old, with hundreds of avatars. Coupled with the fairly small size of the world/fast travel times, there should have been a significant enough amount of interbreeding to have large numbers of benders outside their native lands. However, all the evidence point to this not being the case, even though the bending status of Aang and Katara's children indicates that bending is hereditary. However, this can be reconciled if bending is only hereditary for a single generation. Take Aang and Katara's children. One of their children is an airbender, one a waterbender and one a non-bender. Because the airbender only has airbending children, it's fair to assume that he simply can't have waterbending children.