Not everyone will agree with some of the major changes 2.1 brought into the table, with the most relevant victims being the level revamps and the new NiGHTS-styled Special Stages. 2.2 is currently scheduled to have an entirely new Deep Sea Zone as well.
Some also dislike the new Knuckles, dubbed "Chuckles" due to his idle animation being the mischievous laugh from Sonic 3 instead of him punching in place. Also considering the clash of styles between the current Knuckles and the older Sonic and Tails skins. Sonic is scheduled to receive a remade sprite for 2.2 to match Knuckles' sprite.
The removal of Circuit maps has left a sour taste in the mouths of some. While there are some that enjoyed them, others feel like they needed to be removed as Sonic players who thok forwards repeatedly gave them more of an advantage compared to Tails and Knuckles players.
The change of the Black Eggman/Metal Robotnik battle may also be this depending on who you ask. Some prefer the new battle feeling more climatic over the older version while others preferred the original for being more fast-paced.
Epic Riff: Both acts of Techno Hill Zone use a very distinctive keyboard riff. This riff is what ties the two act tunes together; besides the percussion line and the general musical style, virtually everything else about the music is differentnote Each act uses a completely different bass line, for example.
Fridge Horror: When a Deton crashes into a wall, the animal is released. But in earlier versions, if it hits the player or another Deton, there is no sign of the animal inside.
Tails can easily fly through irritating bottomless pit sections... But even he has trouble in Egg Rock Zone.
Knuckles's gliding and climbing abilities can similarly be used to bypass a lot of platforming sections. It's actually worse with him, since the levels are rarely, if ever, designed against him, and there's no timer for either of his abilities.
If you master it, the Rail Ring effectively becomes one in multiplayer, since it can be fired from any distance making instant contact, and since it only requires one successful hit to make an enemy drop all his/her resources. Plus, the amount of Knockback one character takes from being damaged by a Rail Ring is more extreme compared to those of the other ring types.
Sonic can be this in multiplayer, especially races. Due to an oversight (specifically, the "thrust factor" each character has), Sonic doesn't actually have any disadvantages to his speed, and accelerates faster than Knuckles and Tails. Even the disadvantages he's supposed to have - slippery controls and slow acceleration - are negated by his thok move, which instantly propels him above top speed in any direction.
Sharps. They are blue hockey-puck things that turn red with a loud noise when they see you, and then they chase after you at very fast speeds while invincible, and the whole "get hurt and attack with temporary invincibility" thing won't work. They turn blue again after a while, but you're probably still collecting your spilled rings.
The Buzz enemies from Techno Hill Zone are also fairly annoying, with their tendencies to pop up out of nowhere and knock you off platforms.
Jetty-syns are infamous for their ability to harass players with their unpredictable flying patterns and tendency to attack without warning. Their appearances have been reduced with each update, to the point where they now only appear in Egg Rock Act 2 and Aerial Garden.
Goddamned Boss: Techno Hill Zone's boss. He flies around in circles, splattering around a lot of harmful goo that takes a very long time to disappear. If you don't defeat him quickly, you're going to have a hard time stepping around the gunk. It's also difficult to hit him because he may suddenly change directions as you jump.
What makes him a Goddamned Boss instead of That One Boss is that once you calm down and figure him out, you can easily kill him by standing still in the right place and jumping.
Never, ever, ever type "TUNES 20" into the console. Just don't.
The original intent for Space Countdown sectors was to turn off all sound effects and change the music to TUNES 20. This was cut for numerous reasons, Nightmare Fuel being just one of them...
Incidentally, the sound effect came from MDK2, during a brief trip outside your pressurized space station, the Jim Dandy. It wasn't quite as terrifying there.
The infamous drowning theme makes a comeback and is, as usual, quite terrifying.
TUNES EF, a creepy remix of Greenflower apparently intended for "Evilflower Zone". Whether it's a simple Easter Egg or upcoming content is yet to be seen.
The laugh of the final boss, Brak Eggman, can easily be this. And if that wasn't bad enough, its death cry is even worse!
Sequel Displacement: So much to the point that the original Sonic Robo Blast doesn't have its own page on TV Tropes.
Squick: The intro added in 2.1, where Eggman eats an entire truckload of lard.
Surprisingly Improved Sequel: The original Sonic Robo Blast was among the first Sonic fangames created, and a very ambitious one at that. It was also very primitive (the graphics were all made in MS Paint) and glitchy (it was made in Clickteam's "Klik & Play" and "The Games Engine" engines, both of which were early game engines and very primitive compared to the "Multimedia Fusion" line of engines).
Egg Rock Zone. Justified, in that it's supposed to be preceded by two more acts for pre-existing zones, another zone, and a possibly one-act zone. As it stands, the absence of those four zones makes it a jarring Difficulty Spike. The game is still in development, but priorities shifting to the multiplayer aspect put single player stages on low priority. But even after that, ERZ will still be the hardest level in the game, unless it somehow turns into a Breather Level...
Word of God has stated that ERZ is supposed to be that hard.
Nimbus Ruins Zone, the tenth and final Capture the Flag zone, is perhaps the most difficult of all the current CTF stages, since it is a series of platforms floating above a bottomless death pit. Since flag carriers cannot use their special jump abilities (thoking, flying, or gliding), they must rely on nothing but standard jumps to return to their base.
WTH, Casting Agency?: Because the dev team couldn't fit it in, Tails and Knuckles don't have their own Super Form sprites for the NIGHTS special stages. Instead, They are forced to morph into Super Sonic and use his sprite. Overlaps with They Just Didn't Care.