Greenflower Zone:
For an intro zone, these three levels aren't that bad. Though, GFZ1 is probably a bit too linear. As for the new tunnel at the end of GFZ1, frankly, it looks a bit tacky, even if it's supposed to be a "Look here! You need to go this way!"
And don't get me started on the whirlwind shield room. That's pretty hidden.
GFZ2 has gotten a couple aesthetic touch-ups with some slight modifications to the paths as well, though I still don't like the alternate path at the beginning. It's still too confusing. As for the split path at the end, I do like the outdoors feel to the lower path, but that's about it. The double usage of the springs is cool, but if you strafe away from the second spring, you have to go all the way back to the first, which wasn't an issue with the first set of diagonal springs.
GFZ3... *whistles* I think this might have gotten a bit too difficult. The laser attack is nice, sure, but it's a lot more devastating than it was before. The pinch mode? Unless you hit Eggman fast enough, it can become a bit tedious to even fight him in this phase due to the short window you have.
Techno Hill Zone:
Good lord this place got quite an overhaul. THZ1 looks a lot more industrial in the earlier parts of the level than it was before (I mean, glass barriers? Really? That just doesn't fit there). Though... the third to last checkpoint has a split path that seems to lead to a dead end on the left side, which is really annoying to new players (unless they're playing as Knuckles, but even then it's a bit confusing).
THZ2 has gotten an extension, which looks rather nice, but at the same time a little bit confusing, unless the player actually understands how the slime works from THZ1. Other than that, it's not too bad, minus the fact that the level feels a bit too long for an early level.
THZ3 is probably trivial compared to GFZ3 now with the glass windows, as the slime is the only thing that can hurt you, unless you count the spring below Eggman, which can hurt you... somehow.
Deep Sea Zone:
I still hate water levels, and this isn't that much of an exception. Especially since some paths are puzzle paths. Ugh. DSZ1 is filled with a bunch of open rooms with very little decor outside of the water areas, which is a bit weird. DSZ2 has less of a problem, but it's still rather easy to notice there as well.
DSZ2... well, I really don't know what to say. Personally I like this better than the first act, but that's not saying a whole lot. It's only better because there's a bit more pacing to the level.
DSZ3, if you're going for a fast time, is all about luck. Though, the fact that Eggman zaps the water twice when you're in it is really strange, as you're still invulnerable when the second one hits.
As for the pinch mode? Well... again, luck based, but if you're not afst enough on the 3rd hit, Eggman just drops after one shot, which can be a bit annoying.
Castle Eggman Zone:
CEZ1 starts you off with a huge pit, okay. Other than that, this level's quite ni--wait why is Knuckles' path so hard. His previous path from THZ1 was completely bland, and this one requires you to be nearly precise. And to top it all off? You can't see the spike ball coming at all unless you zoom the camera out.
CEZ2 has gotten quite a big extension since 2.0, which is nice, but the level feels a bit strange once you get into the library part of the level. I don't know about you, but seeing that there just feels a bit strange.
CEZ3 is a rather nice fight, but it's really slow at the same time. Also, I don't like the pinch phase at all. It's unclear what you're supposed to do, and if you mess up on the last hit and fall down, you're basically stuck down there.
Arid Canyon Zone:
All I have to say is, those ... things you have to ride on to get across is probably the WORST part of the level. Especially since you have to do about 12 of those over a death pit. Fortunately, you do have platforms below to la--god dammit I missed the jump to the next platform again. *sigh*
Yes, getting from platform to platform down there as Sonic is a biiiiit too hard.
Red Volcano Zone:
RVZ1 is still an interesting level, especially for where it's at. Personally, my only complaint is the fact that some of this stuff kinda blends in. Not as bad as GFZ2's alt path, but it's still there.
Also, standing on lava? You can't stand on water, so why are you able to stand on lava? That just doesn't make sense to me. Then again, I think that was intentional because otherwise you'd be constantly getting hit every 5-6 seconds in the lava.
Egg Rock Zone:
ERZ1 starts you off at a reasonable pace in terms of difficulty, though at certain points it can get a bit annoying, or even confusing. I don't really have much to say here, though.
ERZ2... oh man, talk about gauntlet. This level is probably the hardest one out of all the main levels. Though, I have to be honest, even though it's the last zone of the game, it might need a slight nerf in a couple areas.
As for the length of the level, personally I think it's fine. Though, if the level was any longer it would be too annoying.
ERZ3... ugh. While I do like the fact that this is here, I don't like it. The race is cool, but after that? Good luck, because Metal Sonic is a bit more unpredictable than he needs to be. He bounces around randomly, his attacks are random, and there's little way to actually dodge said attacks. The only one that can really be dodged is his pinball-like attack. The other two (or however many attacks he has) you can barely tell which one is going to be used, and even then, dodging them is a pain.
ERZ4, also known as ERCZ. In terms of final bosses, this is rather trivial compared to ERZ3, which I guess is alright considering that 2.0 was really confusing to me. If anything, the ring count should be lowered to 2 or 3.
Special Stages:
There are two things I'm curious about: Why use NiGHTS mechanics for the special stages? And why is it that whenever you take a hit, not only do you lose 5 seconds, but you lose an additional 2-3 seconds just to recover? That's a total of 7-8 seconds!
Floral Field:
This is quite a nice way to start off the special stages, as it gives the player an easy start to the levels, which is a good thing, especially since this level resembles GFZ.
Toxic Plateau:
Out of all the special stages, I like this one the most, but I'm not too sure why it stands out to me. Might be all the good aesthetics used in this level compared to the others.
Flooded Cove:
From the early-mid levels, this is probably one of the harder ones with some of its interesting mechanics that can throw people off. Though... just like DSZ, this level feels rather bland.
Cavern Fortress:
I like this level as well, especially since it has a really nice theme to it. Plus it's the first level to really introduce obstacles that can cause you to waste time (unless you count Flooded Cove's water mechanics towards the end, but those are hard to even get hit by). Though... this level feels way too similar to the circuit map that somehow escapes me. Slumber Circuit?
Dusty Wasteland:
Frankly, the name is kinda fitting, especially since this feels completely wasted by the fact that there's Spring Hill Zone as well... and this is basically that but retextured and with a much lower timelimit.
Magma Caves:
This is when the gauntlet really starts kicking in, as this is where you really have to pay attention to the surroundings. My only gripe here is the fact that the level is actually kinda slow, especially with the large room with the spinning fire.
Egg Satellite:
Ow. This one's a doozy, but I guess it kinda makes sense as the counterpart for this is ERZ. Fortunately it's not the hardest, but if you're not careful, this level can be really difficult. I do like this level, though, simply because it pushes your abilities.
Black Hole:
...No comment.
Spring Hill Zone:
There's not really much to say here, other than the fact that there's probably a slight bit more to explore in this level as normal Sonic.
"Hyper Stages":
Why are these called hyper stages?
Aerial Garden Zone:
After putting off completing this as Sonic for years, I finally did it here. I have to be honest, though, this level is rather tedious, but even harder as Knuckles than Sonic or Tails, which is really weird. Might be because he's forced to go through a linear path. Not sure.
Azure Temple Zone:
...ow. As if DSZ was bad enough, this is even more tedious. Fortunately I saved my sanity by going through the gargoyle puzzle instead of the other room... but then again, this level pulls the SAME thing as DSZ: Puzzles. No other zone does this last I checked. Not even CEZ2.
SRB1 Remakes:
These are okay to go through if you just want something to pass the time. Though, I have to be honest, one of the levels is a LITTLE too hard, simply because you can't really see anything in one area. I can't remember which level, though.
Pipe Towers Zone:
Honestly, I liked the original SMB levels. This one is a lot harder, for one, and it's also a bit of a "what a secret?" type of level, as every secret ends up in a dead end.