Pokémon Red and Blue/YMMV

YMMVs that apply to Red and Blue

 * Breather Boss: Bruno of the Elite Four can be easily taken out by any decent Psychic type. This was true in later generations as well, but because Psychic Pokemon were no longer Game Breakers players were less likely to have them in their teams, as well as trainers in general getting smarter and putting coverage moves on their Pokemon. Even without Psychic types, Bruno isn't anything special thanks to his Hitmonchan's elemental punches being hilariously weak and the fact that he carries not one, but two Onixes.
 * Creepy Awesome: Everything about Lavender Town.
 * Demonic Spiders: Anything that had Wrap and was faster than your Pokémon was this. All the opponent has to do is use Wrap over and over to prevent you from attacking. (AI players have infinite power points, you don't) This was especially bad with Tentacool and Tentacruel, who could couple it with Poison Sting, doing more damage and potentially losing a Pokémon.
 * Fridge Logic: When you thwart Giovanni's plans to take over Silph Co and use its technology for Team Rocket's fossil operation, he swears revenge. But when you beat him in a gym match, which has nothing to do with Team Rocket, he disbands Team Rocket and leaves you on good terms. The hell?
 * Also...how the hell did no-one remember Giovanni was once a gym leader!?
 * Because only the player character KNOWS who the leader of Team Rocket is? Outside of Team Rocket, anyway.
 * No, the Silph Co. president and his secretary saw him in person, an NPC in Saffron City mentions that they saw the boss of Team Rocket escape in his helicopter, and the guy posted at the start of Viridian Gym later says after you beat Giovanni "Blow me away! GIOVANNI was the Gym Leader here?" like it's a big deal because Giovanni is notorius for something else. It seems most people knew Giovanni as a mob boss but nobody who might have challenged him in the past ever spoke up about him being a Gym Leader.
 * 1. You are always going to be there to stop him, not matter where he is, so if he causes trouble, he'll be stopped if he's not strong enough.
 * 2. He is out of his gym a lot planning stuff for Team Rocket. Also IIRC it was an old man who can't remember what/who Giovanni was/looked like, so it's likely old age is against him mentally.
 * Though that still doesn't explain the guy at the gym, who's not an old man.
 * Game Breaker:
 * The Psychic Type. On top of being an Infinity+1 Element, the move "Psychic."
 * The item duplication glitch in Red & Blue, which makes the game obscenely easy if used on items like Master Balls and Rare Candies (as it nearly always was). It could also be used to allow one to catch Safari Zone Pokemon the normal way. Fixed in Yellow and the remakes.
 * Pretty much any fast Pokémon with access to Slash, thanks to the gen 1 mechanics that made critical hits reliant on base speed, making Dugtrio and Persian ludicrously powerful.
 * If you used X-Accuracy, all moves got perfect accuracy. And yes, that also includes the one-hit kill moves. After defeating Giovanni, just slap Fissure onto a Dugtrio and laugh as you ruthlessly slaughter everything in your path.
 * Genius Bonus: The Magikarp line is a reference to a Chinese legend that states that if a carp can jump over the Dragon Gate (believed to be at the top of various waterfalls), it will be transformed into a dragon as a reward. This holds more ground when Snap shows a Magikarp jumping into a waterfall and emerging as a Gyarados.
 * Growing the Beard: Pokemon Yellow actually is a better game than Red and Blue in terms of gameplay, as the majority of bugs, glitches, and imbalances are fixed, and the way is paved for the full beard-growing of the franchise with Gold and Silver.
 * Good Bad Bugs: Missingno., M, Glitch City, the Safari Zone glitch, and the Mew glitch, among many others. These games pushed the Game Boy cartridges to their limit, and the majority of the more famous glitches stem from shortcuts the programmers took in order to be able to fit the game within the relatively tiny amount of memory available. The Missingno and Glitch City bugs are the result of the games having no actual memory protection, so the game would try to create a Pokémon encounter or environment from the wrong sets of data with no fail safe to notice the difference. As a result, it's possible to break the game to ridiculous extremes and still have it keep on trucking through everything. Additionally, a bug lets Psychic Pokémon be super-effective against Ghost instead of being completely ineffective (All Ghosts in Gen I were part Poison). This made Ghost Pokémon rather weak but Psychic Pokémon (like Mewtwo) awesome (see Game Breaker above).
 * There are still tons of little ones all throughout the games, one example being that if Hyper Beam KO'd a Pokémon, it did not require a recharge turn.
 * If a Pokémon protects themselves from a Self Destruct/Explosion with a substitute, the substitute disappears, but the Pokémon's sprite never reappears which effectively makes them invisible. Otherwise, everything goes as normal.
 * Sleep-inducing moves used against a Hyper Beam user that is recharging will never miss as well as override any status they have.
 * Ghost is supposed to beat Psychics, but instead Psychics are completely immune! Not that it matters, since the only ghost moves were the wimpy Lick and fixed-damage Night Shade.
 * If Toxic was used to poison a Pokémon, then Leech Seed will inflict extra damage as well.
 * If Haze is used, the opponent will no longer be afflicted of any status effects except Poison, though Badly Poisoned will become normal Poisoned.
 * Depending on their HP value, a Pokémon's recovery moves can fail.
 * Any move that ups the critical hit ratio actually lowers it.
 * Poison is the only status afflictment that Substitute blocks, all other ones get through. However, the substitute hurts itself instead of the user, so that helps a bit.
 * Status buffs that exceed the cap will actually lower the stat.
 * If a Pokémon hurts itself in confusion or is paralyzed while in the air or underground, they remain invulnerable but can still attack unless Dig/Fly is used again, or they're switched out.
 * Harsher in Hindsight: In the Pewter City Museum, there is a model of the Space Shuttle Columbia, which disintegrated in 2003.
 * Hilarious in Hindsight: A Lass outside of the Rock Tunnel complains that there should be a pink Pokémon with a floral print. Now that Black and White is out.... Given the reported number of unused designs for Pokémon (many would get used later), it could easily have been an In-Joke at the time.
 * Nightmare Fuel: A surprising amount for such a light-hearted premise, but Lavender Town and Pokémon Tower are pretty much Nightmare Fuel central thanks to all the creepy rumors and urban legends, as well as the canon horrors such as the ghosts of vengeful Pokémon, Team Rocket callously murdering Pokémon, possessed Channelers, a creepy kid seeing a white hand on your shoulder, and that goddamned music. The logs left behind in the ruins of Cinnabar Mansion where Mewtwo was created detailing its birth and rampage are also pretty damn chilling.
 * Nostalgia Filter: There's a fair base of the Pokémon Fan Dumb as well as the spawned Hate Dumb (towards the newer gens) who refuse to admit that there were ever any other Pokémon, or see any flaws in these games. Many of the game's fans are all too willing to overlook the high amount of Game Breaking glitches (that actually don't require some form of strategy guide to trigger), and some hideous imbalances. Not to mention that there's some Fridge Logic involving the people who refuse to admit that there were new games, as by the time they probably got into the series, Gold and Silver were already being developed or released in Japan!
 * Obvious Beta: While not all the glitches were bad, the fact that they were present at all points to a mild version of this trope.
 * And, of course, the REALLY BROKEN versions were Red & Green, released only in Japan. The games released in America were based on the "Blue" release, which actually fixed some bugs and improved the graphics of the original pair.
 * Seinfeld Is Unfunny: Despite causing the craze for more Pokémon games, a lot of people write off these games as being simplistic and hideously buggy by today's standards.
 * That One Boss: Lt. Surge in Yellow carries only an overleveled Raichu that can not only demolish everything that resists it with STAB Thunderbolt, but can dust the Diglett that you thought you were smart to bring with Mega Punch and Mega Kick. It's telling that he doesn't need any more Pokémon to hurt you with.
 * Erika, because of her over-use of the move Wrap, one of the most obnoxious moves in the first generation. It became a far less troublesome move in later generations.
 * Due to the previously mentioned overpowered Psychic-types and the fact she has a powerhouse of a team in both, Sabrina was a lesson in frustration in the original games. On the other hand, it's entirely possible to accidentally Sequence Break if you couldn't figure out how to get into Saffron and fight her much later than you're expected to, resulting in a somewhat easier battle, especially if you've got Surf.
 * Koga in Yellow's level 50 Venomoth can be a nasty surprise for players not expecting such a high level jump. Even with proper grinding, his team is still an absolute pain thanks to Toxic + Double Team trickery.
 * Unfortunate Implications: Upon arriving in the US, Jynx caught some flak for being a blackface caricature (the intention may have been a ganguro or a type of ice witch instead, but we're not sure). Its skin was changed from black to purple in later games to avoid problems.
 * Villain Decay: In the Yellow version, Jessie and James are never all that tough to beat (except maybe in the first encounter), but the way in which they speak to you gets progressively sillier with each encounter, that by the last time you face them, they don't even threaten you so much as they casually ask you to not "disturb their boss' meeting."

YMMVs that apply to FireRed and LeafGreen

 * Accidental Innuendo: Ever take a good look at the fountain at Silph Company first floor?
 * Base Breaker: Similarly with Lyra, Leaf has her own segment of Hate Dumb that give her this. Arguments on whether she exists or not are best left avoided.
 * Memetic Molester: Hypno got this worse than ever thanks to the one harassing Lostelle inside Berry Forest. Seriously, try looking anywhere for anyone who doesn't make the "Hypno = Pedo" connection.
 * That One Sidequest: The goddamned Lucky Egg. Do you want to make it to where your Pokémon get an EXP boost during level grinding for the Pokémon League rematches? Well, you just have to look around the Safari Zone where Chansey is already the rarest thing there, hope that it doesn't run away (which they are VERY prone to doing), and hope and pray that it was carrying the egg. How likely is it to be carrying that egg? A measly 5% chance. It's very likely that even the most dedicated Chansey hunters won't ever find one in their lifetimes.
 * They Changed It, Now It Sucks: A common response to the Extended Gameplay and Sevii Islands.
 * Although the Sevii islands may be the Scrappy Level even if you do like the extended playtime. They do have their fans, however.

YMMVs that apply to both the originals and the remakes
The Woobie: The Cubone in Lavender Town whose mother was murdered by Team Rocket. It's hard to find anyone who doesn't want to give the poor little guy a hug after what happened to him.
 * Accidental Innuendo: Prof. Oak. He came when he heard beat the Pokémon League.
 * Alternate Character Interpretation: Is Blue simply acting like a douche to get Red to improve his skills as a trainer? Or is he simply an out-and-out asshole? The latter is blatantly obvious, but there are still plenty who believe the former.
 * Another bit going towards Blue is the infamous Raticate incident. He has a Rattata that's later a Raticate on the S.S. Anne, but its never seen again after that battle... and the next time you run into Blue just happens to be at the Pokémon Tower. Did you heartlessly kill his beloved Pokémon, or did he simply box it (or even release it) because he didn't find it useful anymore and you just happened to run into him there? Like before, the latter is obvious, but lots of people tend to think the former is true.
 * Anticlimax Boss: You'd expect to be given one hell of a show whenever you run into Giovanni at the Viridian Gym, where he just so happens to be the leader... but his whole team consists of Ground types, and they can all be dusted in one shot by a decent Water move, and since you need Surf to get the seventh badge, you will have access to it whenever you fight him. Even though he carries a on-hit kill move in the form of Fissure, he still is ridiculously easy to beat.
 * Base Breaker: Rival Blue tends to fall into one of two categories. He's well loved for being viewed as a perfect rival and a Badass while he's simply reviled by others and viewed as simply an obnoxious loser.
 * Pikachu. Is it an adorable little mascot who deserves the attention it gets? Or an obnoxious rat that gets way too much over-exposure?
 * In spite of its Ensemble Darkhorse status, Charizard also gets a lot of ire from those who view it as horrifically overrated, over represented, and just plain shoved down people's throats. Its status as the Genwunner's favorite does not do its perception any favors.
 * Dragonite tends to have fans split into two camps: those who find it absolutely badass and adorable to boot, and those who find it overly goofy and stupid looking as well as overrated compared to later dragons in the franchise.
 * Big Lipped Alligator Moment: The whole incident with Bill. He gets accidentally turned into a Pokémon in an experiment gone wrong, and you help change him back. He then gives you the S.S. Anne ticket, sends you on your merry way, and this is never, ever bought up again. Ever.
 * Breather Boss: Giovanni is rather easy for being the final Gym Leader, since most of his Pokémon are rather slow and all of them have common weaknesses. Or DOUBLE weaknesses.
 * Even more so in Fire Red and Leaf Green, where his strongest Pokemon in the original Red and Blue--Rhydon--was replaced by a Rhyhorn of all things.
 * Draco in Leather Pants: While he's not an evil character, Blue still has an inexplicable amount of fans who view him as being a likable guy despite his absolutely boorish and callous attitude. Many people are somehow convinced that he's lashing out at Red because Red is somehow a terrible person and that Blue himself is a saint in comparison, and view Professor Oak as being in the wrong for telling him off after being defeated at the Pokémon League... despite the fact that there is absolutely no evidence for the former and that he totally deserved the latter.
 * There's also a number of fans who wish that you had the option to join Team Rocket, the mafia/yakuza group dedicated to killing Pokémon and stealing from others. Chalk that one up to Evil Is Cool.
 * Ensemble Darkhorse: Brock, though mostly because he is an Ascended Extra in the anime.
 * Misty even more so. Was made into a major character in the anime and Pokémon Special.
 * Don't forget Blue/Green/Gary, the widest loved rival in the franchise.
 * On the Pokémon side of things, we have Pikachu, Charizard and Mewtwo.
 * Eevee is beloved for its unique gimmick (evolves into three different Pokémon, and later up to eight through different methods), cute appearance, and versatility.
 * Butterfree and Beedrill are not the most useful Pokémon out there, but they still have tons of fans. In Butterfree's case, the anime probably helps.
 * The Pidgey line is also well-remembered thanks to being quite reliable for pretty much the entire game despite being a tad mediocre in general.
 * Both Psyduck and Slowpoke tend to be among the most beloved Water Pokémon for their rather dopey yet loveable appearance.
 * There's also the almighty, ALL POWERFUL MAGIKARP! Despite being ridiculously weak, its beloved exactly for those reasons, and its evolution is unironically loved for being ridiculously badass in its own right.
 * In a game full of weaksauce Bug types, Scyther and to an extent Pinsir were well-liked thanks to the fact that they could at least hold their own in battle, albeit not as good in later games.
 * Arcanine gets this a lot too, thanks to being a fantastically majestic Pokémon with access to some pretty good moves. The fact that it's a Big Friendly Dog helps a lot. Its Blue-exclusive counterpart Ninetails gets this as well for being rather pretty and even a tad creepy.
 * In general, Pokémon that both look really cool and did well in battle like Starmie, Alakazam, and Gengar got this treatment.
 * It Was His Sled: The fact that Giovanni, the boss of Team Rocket, is all but common knowledge among even lesser fans who have never played the game.
 * As is your rival.
 * Love It or Hate It: Thanks to the tensions between Genwunners and fans of newer generations, it's hard to find middle ground regarding the quality of these games. People tend to either view Gen 1 as being the best region with a fantastic rival who inspires you to do better, simple yet memorable Pokémon designs that make sense, and gameplay that while not perfect, is still simple and effective enough, while others view gen 1 as having a horrifically boring region with 1 dimensional uninteresting characters, an obnoxious rival who goes through no character development who is a pathetic joke, Pokémon designs that are boring and uninspired, and gameplay that is horrifically buggy to the point of unplayability. Good luck with convincing people that you like it as well as other generations!
 * Magnificent Bastard: Giovanni is this, albeit less so than later villains like Cyrus and Ghetsis (to a point). Despite being a criminal mastermind, he still treats Red with respect and is always well-mannered and polite even whenever disbanding Team Rocket. There's also the fact that he lives a double life as the Gym Leader of Viridian City, a fact that few people are even aware of and continues to run crooked operations in Celadon City right under people's noses.
 * Memetic Badass: Both Red and Leaf, actually. More so Red than Leaf, due to him being more well-known. Leaf gets her fair share though, when she's not on Red's level she's usually shown as being the best of the trio in both a playful manner and a battling manner.
 * Memetic Outfit: Red's original outfit. Just try looking for fanart of Red in his new look outside of FRLG artwork, especially HGSS artwork (which is odd when everyone else has their HGSS outfits). That includes his hair. Not even in HGSS artwork do you see him in his new one, despite the fact he is explicitly shown in both his new hair style (overworld sprite) and his new outfit (battle sprite).
 * "Hi. I like shorts! They're comfy and easy to wear!"
 * Moral Event Horizon: Team Rocket as a whole is revealed to have crossed it by killing Pokemon whose souls now reside in Lavender Tower, including Marowak.
 * That One Boss: Misty and her overpowered Starmie. In the re-release, it is level 21. If you didn't take Bulbasaur, you're in for some power grinding for your whole team. And it's not even as hard as it COULD have been, had it been given a psychic move to match its subtype. All the available grass types are subtyped poison at this point in the game. Not Paras though... But then again, the only way to get a Grass-type move on it at that point is with the Bullet Seed TM in the remakes.
 * Sabrina in both games, even with the massive nerfs Psychic got in later generations thanks to using Calm Mind to jack up her Pokémon's Special stats to ludicrous levels.
 * Unwinnable by Insanity: Unleash all of your Pokémon. Sell all your Pokéballs. Leave yourself with only one Pokémon who can't use Surf or fly. Then blow all your money on junk that you can't actually use to obtain new Pokémon. Save your game while on Cinnabar. Now you can't win the game! This is one of the few ways to make the game completely unwinnable through legit methods, and back when you could rent this game, it couldn't be uncommon to find a game that had been left with the save-file trapped in Cinnabar.