Anti-Climax Boss: For many fans, the final boss, once you know the trick. Lure it to one side of the room, hit it before it recovers from the recoil of its stomp attack, and retreat to the other side of the room before it shoots its spiked arms at you. Rinse and repeat. No joke, this was the intended way to fight it; your biggest enemy is your own nerves, being the final boss and you have no rings. Similarly, the boss will be a cakewalk if you can get behind him and stay there. When behind the mech, it has no offensive moves except when standing still, leaving you free to hit its jetpack unopposed. He can't shoot his arms at Sonic, and he can't turn around. He can shoot bombs at you, but they drop in a set pattern, and only when he stands still.
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.
Chemical Plant Zone is a common favorite, due to being one of the fastest levels in the Genesis games. It's in Sonic Generations, Sonic Mania and Sonic Forces for a reason! The other favorite is Casino Night Zone, which codified the Casino Park style level for both the series and for subsequent games. It was in the 3DS version of Generations instead.
Hidden Palace Zone in the remake.
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.
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.
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.
Demonic Spiders: All the badniks in Metropolis Zone are Goddamned Bats, but the Slicer badniks stand out the most. 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, have the annoying ability to home in on Sonic, and are usually placed right in trial-and-error spots. 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?
"The Unholy Trio" for the Metropolis Zone badniks.
"Silver Sonic" or "Robo Sonic" for the Sonic robot fought in Death Egg Zone. Its official name is "Mecha Sonic", but this creates confusion with the distinct robot in Sonic & Knuckles that shares the same name. "Silver Sonic" is taken from the 8-bit Sonic 2 manual (which actually describes yet another distinct bot; see Fanon below) and was used in the Archie comics, while "Robo Sonic" was used in its non-canon LEGO Dimensions appearance.
The almighty speed booster is introduced in Chemical Plant Zone. Not a problem here, but it caught Dimps' eye, leading to a common criticism of the Sonic Rush series and especially Sonic the Hedgehog 4 being the sheer overabundance of them throughout the game.
Sonic 2 would start the trend of every major title introducing at least one new character, in this case Tails. This initially wasn't a problem since Tails lacked the common writing problems and Gameplay Roulette that plagued future characters, but with each game that passes by with a new residential Base-Breaking Character or Scrappy, the novelty wore off and now any implication of adding a new character in the latest Sonic title can cause fans to lose it.
Game-Breaker: If you plug in the second controller, Tails can be used to defeat Eggman's machines easily because of his invincibility.
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 Metropolis Zone. Asterons, suicidal starfish bots that shoot spikes when they blow up, Slicers, praying mantises that throw their pincers like a boomerang (not at all helped by their cheap placement), 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.
Sonic can't be hurt by the otherwise nasty Water Eggman boss in Chemical Plant Zone 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 Spin Dash before you get launched, it will carry over and you'll shoot forward as soon as you hit the ground. This became an Ascended Glitch in Sonic Mania with the introduction of the Drop Dash. This video of Sonic 2 Retro Remix plays it to hilarious extents.
Harsher in Hindsight: Oil Ocean Zone (a level in which the water is made entirely of oil dumped by refineries) becomes a lot more tragic when one accounts for real life oil spillages in seas and oceans, in particular the Deepwater Horizon accident.
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.
It's the Same, So It Sucks: Generally every rerelease made after the Android remake has a Tough Act to Follow due to the lavish number of improvements and extras it has. It is also glaringly the only official version of Sonic 2 that allows the player to use Tails' ability to fly.
Nausea Fuel: The iconic half-pipe Special Stages are often called "Puke Zone" for its crude and poorly aged imitation of 3D. The framerate is very low, making the stages look like they're put under a strobe light, and the sudden twists and turns make the effect even more disorienting. Several players often complain of eyestrain and motion sickness when playing these stages. Thankfully, the remake ditches the old version and makes them true polygonal stages, making them much smoother.
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.
Similar to the Sonic 1 rerelease, the SEGA AGES version isn't nearly on the same caliber as the Android remake, but is still an enjoyable package, having flawless emulation while adding Drop Dash and rumble feature as well as including the Sonic and Knuckles lock-on rendition to play as Knuckles.
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.
Recycled Soundtrack: The music written for Hidden Palace Zone and Oil Ocean Zone in the beta became those of Mystic Cave Zone 2P and Casino Night 2P, respectively, in the final version. In turn, a unused song present in betas was used as the final music for Oil Ocean, with the dropped Hidden Palace getting a song intended for an unmade cutscene before being cut. One further unreleased song was used for Emerald Hill Zone 2P. Fan speculation is that the Oil Ocean and Emerald Hill 2P music were intended for planned levels never implemented in the game, though this has never been confirmed.
When leaving a Special Stage, you lose all your rings, which can be frustrating for those who want to get all 7 Emeralds early on, and also for when you exit a Special Stage from a pre-boss checkpoint, as boss arenas often have no rings. While the original version of Knuckles in Sonic 2 has it where Knuckles' ring count when he touches a checkpoint is restored even after he loses a life, this is not retained in the iOS/Android port.
Having Tails tag along in the Special Stages can be frustrating if the CPU is controlling him, as you lose rings every time he gets hit. Even worse, sometimes he may get in front of Sonic if you hit a bomb, leading to him collecting all the rings and not you, and he'll most likely smack himself into a bomb and lose them all.
Changing to Sonic's super form is triggered by jumping... and nothing else. And since this is a Platform Game, you will be jumping a lot. Hit 50 rings but don't want to transform? Too bad! Thankfully, in the mobile remake, it's instead triggered by pressing the jump button again in midair.
Signature Scene: This game's memorable ending was revisited in a few other Sonic games.
Tear Jerker: The unused track for the Dummied Out Hidden Palace Zone (track 10 in the Sound Test). Somehow it is very sad, and almost sounds like the music for the bad ending of the game (if it had one).
While not terribly difficult, Catcher Eggman at the end of Casino Night Zone is pretty tricky, and it can be hard to beat him with all your rings still intact by the end.
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 Spin Dashing into it, resulting in a lost life. If the fight goes on for long enough, it starts to shoot spikes during its Spin Dash. Also, you'll have to fight it 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.
Chemical Plant Zone Act 2. You know it's a wake-up call when Sonic Team themselves (or 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.
Mystic Cave Zone, 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 (made worse if you happen to be Super with hundreds of rings). This was fixed in the remake, as the spike pit is replaced with a transition to a new level.
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 badniks 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). It is also the longest Zone in the entire game, bearing three Acts instead of the standard two.
They Changed It, Now It Sucks!: While the 2013 remake has received considerable acclaim, some fans have taken it to task for adding in elements that they deride as gimmicks (such as including the elemental shields from Sonic 3), using a slightly modified version of the Simon Wai prototype's Hidden Palace Zone rather than trying to recreate the speculated original vision of it being where Sonic learns how to access his Super Form after collecting all seven Chaos Emeralds, and not using Track 10 for Hidden Palace.
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.
Anti-Climax Boss: Silver Sonic, the penultimate boss (and, if you haven't gotten all of the Chaos Emeralds, the final boss) is probably second easiest boss next the the seal boss. He mainly just jumps around, and if you bump into him you just both go flying back without harm. His only other attacks include simply charging at you (at which point he is completely vulnerable to hits) and attacking with a claw like appendage (which may kill you if you're unlucky or haven't fought him before, but is still pretty easy to avoid as it's reach is very short.)
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.
Breather Level: Green Hills Zone. Seriously, due to its stereotypical "peaceful grasslands" setting and easiness compared to the rest of the game, it's like they cut what was supposed to be the first level and pasted it in the middle of the game. Additionally, it has the most 1UPs and Rings out of any zone in the entire game, with the very first act alone having a whopping four 1UPs and well over 200 rings. This makes (re)stocking on lives insanely easy.note Run through most of the level collecting the goods, die towards the end, and repeat until you're satisfied on lives.And you'll need every last one of them for the rest of the game, too, starting with Green Hills's third act.
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.
There's no canon indication that the 8-bit Silver Sonic◊ is the same Mecha Sonic from the Genesis game◊, but the fandom generally treated them as the same robot out of simplicity. This has shifted a bit with the bot's surprise cameo in Sonic Mania, however, which depicts it at the same height as its 8-bit sprite (and thus, much smaller than the Genesis Mecha Sonic); with this in mind, Silver Sonic is now often treated as a "younger brother" to Mania's Metal Sonic.
There's no official word from Sonic Team or Sega that Tails actually dies in the Bad Ending (or that Eggman was ever going to kill him), yet it's been a very popular piece of fanon for a long time. The theory is contradicted by the good ending, which shows the same image of Tails in the sky (accompanied by Sonic), even though he's been rescued.
Though it's never explicit and largely an assumption made by fans, the bad ending implies that Tails is murdered (or at least still under Eggman's mercy). Several years later, one of the biggest criticisms of some of the later games (in particular Shadow the Hedgehog and Sonic the Hedgehog (2006)) are that their Darker and Edgier tone is incredibly jarring and sometimes frustrating in a series about Funny Animals fighting an evil fat man.
This game introduced Tails as being "weak", as he's easily captured by Robotnik. Fast-forward twenty-five years, and Sonic Forces' story was criticized for portraying Tails as cowering at threats.
Older Than They Think: The 8-bit version actually predates the Mega Drive/Genesis game by two months, and was Tails' 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.
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. This is made even worse by the per-recorded advice given by the Sega Help Line, a phone number printed on the top of the game cartridge in some regions. The advice for Sky High Zone Act 1: "use the hang glider to fly across the gap". Thanks.
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.
Tear Jerker: The bad ending. Even if you don't subscribe to the theory that Tails dies in the bad ending, it still leaves the fact that Sonic went on the journey just to save him from the clutches of Dr. Eggman... and he didn't. The music doesn't help matters that much.
That One Boss: The Mecha Antlion, specifically on the Game Gear. It's considered tougher due to the smaller resolution mentioned above, making it harder to avoid the bouncing balls (you're also on a 90 degree slope that leads to the boss, and falling into the actual boss on the right of the screen is also easy to do). And it's the first boss in the game.* Making matters worse, some bombs fall low and others bounce high, whereas in the Master System version they all bounce low and are easily jumped over. Oh, by the way, after throwing the last bomb, Eggman does a fast ground-sweep charge along the ground that you barely have any time to jump over, plus it's possible for a bouncing bomb to still be coming down the slope when Eggman comes down. Every boss afterward is a pushover by comparison, though you don't get rings for any of them.
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.* While you can jump right through the fake wall to get to the exit, you must take the pipe under it to be able to reach the zone's Chaos Emerald, which is required to reach the final level, and being unable to go back up the pipe after the Emerald will force you to deal with the final shaft with the spikes, spears and Badniks. Doesn't help that it's an enormous level compared to most of the others.
Green Hills Zone Act 3 takes a step way up in difficulty from the other two acts; it forces the player to bounce across a mountain range of springs without landing in spikes at the bottom of the level, which will kill them due to the lack of Rings. The final three hills require near-perfect timing to get past, and losing to the Master Robot after these last three hills sends the player back to the beginning.
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 The Only One Allowed to Defeat You mindset at the time. And then he drops bombs onto the boss... the only thing that can harm it. If he just refrained from attacking, Sonic would be stuck there forever.