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?
 * 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.
 * Franchise Original Sin: The Gamegear version's 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.
 * 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.
 * 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.
 * Nightmare Fuel/ Tear Jerker:
 * The bad ending of the Master System/ Game Gear version implies that Tails is dead. Trauma Ensues.
 * Thought the infamous drowning music only plays when you're about to drown or hit the 10-minute time limit? Well, in 2-player mode, when one player finishes, the other player has 60 seconds to finish the stage or it's TIME OVER for them. So the time limit ticks down to 11 seconds and...yeah, have fun playing 2-player with someone significantly slower or faster than you now.
 * 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.
 * 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.
 * That One Boss:
 * From the Genesis version: Silver Sonic and Metal Robotnik, the penultimate and final bosses respectively, which are fought aboard the Death Egg. Without rings. Silver 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 Silver Sonic once again if death occurs. The latter (along with other bosses) is made even harder when playing as Knuckles (made playable with locking Sonic 2 onto the Sonic And Knuckles cartridge) due to his lower jump height.
 * 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.
 * While not terribly difficult, Robotnik 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).
 * That One Level:
 * For the 8-bit version, Aqua Lake Zone Act 2 could very well contend for the title. 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.
 * Scrambled Egg Zone (8-Bit), 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.
 * Metropolis Zone (GEN), 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.
 * Also, Chemical Plant Zone (GEN). Not as hard as Metropolis, but definitely a wake-up call. Sonic Team themselves (at least, the members who developed the Sonic Gems Collection) had trouble with this level.
 * 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 too, although it's nowhere near as bad as Metropolis Zone, which comes right afterwards. The sea horse and octopus 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 sea horses can also fly at you from out of nowhere.

The Master System/Game Gear version:

 * Breather Boss: In the Master System and Game Gear version, the Seal Mecha of Aqua Lake, who is almost incapable of harming you.