Sonic the Hedgehog 2/YMMV

The Genesis/Mega Drive version:

 * Author's Saving Throw: In the first game, Marble Zone was criticized for its slow pace and focus on tricky platforming. Chemical Plant Zone was therefore designed as Marble Zone's antithesis, with the game letting the player use Emerald Hill Zone to get to grips with the controls and playing style, and then letting them go all-out when they reached Chemical Plant Zone. Granted, while the zone still has one or two tricky bits of platforming, they are just a minor speedbump instead of taking up the entire level.
 * Awesome Music: A returning Masato Nakamura strikes gold again with the Genesis version. The 8-Bit version has some good music too, and the music from Green Hills Zone would be adapted into "Sonic - You Can Do Anything", the opening theme from the Japanese version of Sonic CD.
 * Best Level Ever: Chemical Plant Zone is a common favorite, due to being one of the fastest levels in the Genesis games.
 * Breather Boss: Mecha Sonic, the second-to-final boss. While it's still a tricky fight (the lack of rings, its small hitbox and variety of attacks not helping your case), with just the right timing, he can be defeated in roughly 10 seconds. If anything, he's just a warm-up for the fight against the Death Egg Robot.
 * Breather Level:
 * Casino Night Zone has lots of opportunities to collect rings, making it a good level for picking up Chaos Emeralds. Downplayed due to its instant death squishing hazards and formidable boss.
 * Sky Chase Zone, an easy and relaxed level with lovely, gentle music as you fly through the sky. Helps that it's after the horror that is the Metropolis Zone.
 * Broken Base: Whether the Hidden Palace Zone in the remake should've used the Dummied Out track instead of Mystic Cave Zone's 2-player music. Some feel that Hidden Palace deserves its own track, while others consider its theme to be Soundtrack Dissonance. The Mystic Cave Zone 2 Player music was always Hidden Palace's music in the beta, but the unused level in the final uses the unused song.
 * Demonic Spiders: ALL the badniks in Metropolis Zone are Goddamned Bats, but the Slicer (mantis) badniks stand out the most as Motherfucking Bats. Shellcrackers can be avoided by jumping over the large claw. While Asterons explode at the worst of times, they have spikes that fly out straight in a star pattern and thus can be avoided. Slicer's claws, however, are quite large, and have the annoying ability to home in on Sonic. Needless to say, they will get you at least once unless you are Super Sonic. Small wonder why many considered it the most hated badnik in the game, if not the most hated badnik in the Genesis-era Sonic the Hedgehog games, and would you like to guess who's back in the fourth game?
 * Disappointing Last Level: The Death Egg. All the effort to reach it, and the final level is just two boss fights with no rings, meaning you are forced into Trial and Error Gameplay until you get their tricky patterns down, which depending on how many lives you have left can force you to start the entire game over again.
 * Ear Worm: The boss theme is very likely to get stuck in your head.
 * Even Better Sequel: With the possible exception of Sonic CD and the combined Sonic 3 & Knuckles, this is usually considered the best-ever Sonic game.
 * Game Breaker:
 * If you plug in the second controller, Tails can be used to defeat Eggman's machines easily because of his invincibility.
 * If you can get all the Chaos Emeralds, Super Sonic makes most of the game a cakewalk thanks to the increased speed and his invincibility. The only level it won't help with is of course, the Death Egg because it has no rings.
 * Goddamned Bats: A few enemies stand out (such as the firefly Flasher enemies in Mystic Cave and the Aquis seahorses in Oil Ocean), but the biggest set of Goddamned Bats live in the Metropolis Zone. Asterons, suicidal starfish bots that shoot spikes when they blow up, Slicers, praying mantises that throw their pincers like a boomerang, and Shellcrakers, crabs with huge fists that always seem to be positioned in awkward places. One of the main reasons why Metropolis is That One Level.
 * Good Bad Bugs:
 * Sonic can't be hurt by the otherwise nasty Chemical Plant boss if he kneels, making it much easier to defeat. Interestingly, this doesn't work if you play as Tails. Also in Chemical Plant, there's one point where Sonic can go so fast that he can outrun the screen.
 * "Super Tails": Use the debug code, enable some means of getting all seven Chaos Emeralds (either code or genuinely) and transform, then generate a Teleport monitor and jump on it. His jump height is still too low to be really useful, and it goes away if he falls into a pit or gets crushed, though.
 * With objects that launch you (such as the see-saws in Hill Top Zone) if you spindash before you get launched, it will carry over and you'll shoot forward as soon as you hit the ground. Demonstrated in this speed-run of Act 1. This video of Sonic 2 Retro Remix plays it to hilarious extents.
 * Harsher in Hindsight: As these two YTMNDs demonstrate, this game features the Gulf of Mexico Oil Ocean Zone.
 * Hilarious in Hindsight: It looks like it says Sonic F on the side of the Tornado, doesn't it?
 * Internet Backdraft: Sega's apparent decision to not release the remake on PC or consoles and keeping it on mobile devices has gotten a bit of hate, not helped by the additional content that will probably remain exclusive to the mobile versions.
 * It Was His Sled: Hidden Palace Zone in the remake is intended to be a surprise to players, but it's pretty much impossible to discuss this in any Sonic community without someone spoiling it.
 * Moral Event Horizon: Robotnik crosses this in the bad ending of the Master System/Game Gear version. It's hard to see him as the same Affably Evil Mad Scientist.
 * Polished Port: The iOS/Android versions has Knuckles integrated in without needing Sonic & Knuckles, a Boss Rush mode, an extended vs. mode, an updated soundtrack, and both a finished version and a secret beta version of Hidden Palace. The game also has a couple new visual effects (most evidently, the Special Stage, which has a very smooth framerate). This is due to being built on a new, custom engine designed to port classic Sonic games.
 * Porting Disaster: The PlayStation 3 version of the game has glitches happen far more often, such as removing hitboxes for some walls that, while you normally wouldn't run into, have springs.
 * Scrappy Mechanic: The infamous hang gliding sections from Sky High Zone, panned for the hang glider controls taking a great deal of time to get used to.
 * Signature Scene: This game's memorable ending was revisited in a few other Sonic games.
 * That One Boss:
 * While not terribly difficult, Catcher Eggman at the end of Casino Night Zone is pretty tricky, and its extremely difficult to beat him with all your rings still intact by the end (tragic considering how many rings one can end up with on this level).
 * Much the same applies to Flying Eggman, the boss of Metropolis Zone (except that, it being That One Level, you've probably lost most of your rings before you reach him).
 * Drill Eggman II, the boss of Mystic Cave Zone, causes stalactites that are rather hard to avoid to rain down from the ceiling. Even so, it's not overly challenging if you have rings... but if you activate the checkpoint directly preceding the boss fight and then either die to the boss or enter a Special Stage from that checkpoint, you'll be forced to do the fight without any rings.
 * The Wing Fortress Zone boss (Barrier Eggman), as you have to watch out for randomly moving spike platforms and a laser all at once, with limited space to move around. Dodging the laser is easy enough, but avoiding taking damage from the spike platforms isn't so easy, and you have to jump on the platforms to reach the laser and get a hit. Have fun if you lose all your rings.
 * Mecha Sonic, the penultimate boss fought aboard the Death Egg. Without rings. Mecha Sonic has a hitbox that's very small and you get no bounceback from spindashing into it, resulting in a lost life. Towards the end of the fight, it starts to shoot spikes during its spindash. Metal Robotnik protects its weak point with its spiked fists, leaving only a small opportunity to successfully hit the weak point. Touching the fists or legs results in death, and you'll have to fight Mecha Sonic once again if death occurs when fighting the Death Egg Robot.
 * While the Death Egg Robot is already difficult normally (with its spiked arms making it tricky to jump into it, the lack of rings against it, and it taking twelve hits as opposed to eight), when playing as Knuckles (by locking Sonic 2 onto Sonic & Knuckles), it becomes a nightmare thanks to his reduced jump height compared to Sonic and Tails, making it easier to jump into the spikes and impossible to sneak over them with a running jump.
 * That One Level:
 * Chemical Plant Zone. You know it's a wake-up call when Sonic Team themselves (at least, the members who developed the Sonic Gems Collection) had trouble with this level. Aside from being a water level (without any air bubbles), there's one segment where you must climb up a series of rotating stair blocks as water rises. You can potentially get crushed, or (heaven forbid) fall back down into the water and have to do it all over again.
 * Some consider Mystic Cave Zone That One Level as well, since literally almost every single object in the zone is a trap of some sort, and the enemies are either hidden or small enough to overlook. Oh, and it has an inescapable spike pit in Act 2.
 * Some players consider Oil Ocean Zone to be one, although it's nowhere near as bad as Metropolis Zone, which comes right afterwards. The Aquis and Octus Badniks that shoot at you are extremely hard to avoid, making the whole level a bit of a headache if you're trying to retain your rings. The Aquis can also fly at you from out of nowhere.
 * Metropolis Zone, mostly due to the enemies there being the toughest non-boss enemies in the game and some are even placed poorly in the level such as one of the Slicers being at the top of a yellow triangle spring wall you have to bounce up to making it hard to avoid the boomerang blades and avoid running into the slicer at the top of it.
 * Visual Effects of Awesome: The 2013 remade version of the game makes the Special Stages true 3D instead of just pseudo-3D (though objects like bombs, rings, and the characters are still sprites), giving it a much smoother and fluid look compared to the Genesis original. Also using a code a secret eighth stage can be played and features a corkscrew effect on the half-pipe that is unlike anything on the Genesis version's stages.

The Master System/Game Gear version:

 * Breather Boss: The Mecha Sea Lion of Aqua Lake is almost completely incapable of harming Sonic, and his only method of attack is inflating an explosive ball to throw at you, which can be very quickly popped before he can even launch it... it can be taken down in mere seconds.
 * Contested Sequel: In contrast to the near-universally beloved 16-bit entry, this one's a lot more divisive, with opinions ranging anywhere from it being a Sophomore Slump to being the best of the 8-bit entries.
 * Franchise Original Sin: Though it's never explicit and largely an assumption made by fans, the bad ending implies that . Several years later, one of the biggest criticisms of the games are that the Darker and Edgier tone is incredibly jarring and sometimes frustrating in a series about Funny Animals fighting an evil fat man.
 * Older Than They Think: The 8-bit version actually predates the Mega Drive/Genesis game by two months, and was Tails's first true appearance in the Sonic series.
 * Porting Disaster: The Game Gear port suffers from the camera being zoomed in on Sonic too much, but not having the levels compacted in order to compensate, leading to a lot of cheap deaths and leaps of faith required on behalf of the player.
 * Sophomore Slump: Downplayed. While not a hated game outright, it's considered far less remarkable than its 16 bit counterpart, largely being a Mission Pack Sequel for the first 8-bit game, with Sonic's third 8-bit platformer Sonic Chaos being a better translation of 16-bit Sonic 2's new innovations and gameplay.
 * That One Boss: The Underground Zone boss in the Game Gear version is considered tougher due to the smaller resolution mentioned above, making it harder to avoid the bouncing balls. And that's the first boss in the game. Every boss afterward is, comparatively, a pushover.
 * That One Level:
 * Aqua Lake Zone Act 2. For starters, it's completely underwater, and any Sonic fan would know what underwater means in this context: drowning. To make matters worse, it has patches of fake walls in rather nasty spots that you can accidentally fall in, and at least four big bubble dispensers you climb into to float upwards. The worst of these is near the end, which can fortunately be avoided by going through a fake wall, but if your speed is not maintained, you fall down and have to do that horrible "dodge the spikes + enemies hoping your bubble doesn't pop" section. Doesn't help that it's an enormous level compared to most of the others.
 * The third act of Green Hills, in comparison to the rest of the zone: it's a poster example of Platform Hell with hills, springs and spikes, with the final approach to the boss being completely unforgiving in its timing. And no checkpoints means you have to do this again if the boss kills you.
 * Scrambled Egg Zone, thanks to the very elaborate and very confusing transportation pipe system, full of Trial and Error Gameplay as one wrong move can mean inescapable spike pits. There's only one pipe maze with spikes that you CAN climb out of near the end of Act 1, but Act 2 has several spike pipe puzzles, and the last one requires timing with no room for error, or it's a spike pit that is effectively a bottomless pit for Sonic.
 * What an Idiot!: Just before Underground Zone's boss, Sonic is sent careening right for a pool of lava... and Eggman saves him from this most-definitely-fatal situation just so Sonic can fight the boss instead. Presumably, Eggman had a The Only One Allowed to Defeat You mindset at the time.