I played this through Story Mode from Sonic the Hedgehog 1 to Sonic the Hedgehog 3, Zone 2: The Scottish Play. I can confirm that Tails not tracing properly and just jumping around loudly forever is my number one complaint. Also, I finished one zone, did a little Tails-flying as a sort of "victory pose"... and the Tails helicopter sound looped forever for the entire next stage. Not great. Also, I haven't played much of Sonic the Hedgehog 3 & Knuckles yet (again, quit at second zone for the night), but it seems like the graphics are a little... weird? I don't know. It doesn't look as smooth as the other games. Like maybe it was already an attempt at being "better" on the Genesis, and it wasn't upgraded as much as 1, CD, and 2? I don't know. I don't think I ever sat down and played through the series as immediately sequentially before. Oh, and the wider screens or something now causes some of the projectile-based badniks to spawn at inopportune times. I have definitely been "sniped" by more than a few Buzz Bombers that were way the hell off in the distance.
Despite my reservations earlier in the thread, I did not notice the boss arena widths as much as I thought I would. I mean, I noticed, but it wasn't too bad anywhere but in Spring Yard.
Good things:
1. I like that you can now easily enable some Tails flight with Sonic 1-player if you so choose.
2. I like that Super Sonic is on its own button, so you don't accidentally activate it whenever.
3. Sonic CD is the game I am least familiar with. I think I played through that once before on the Gamecube. That said, it seems very pretty and pleasant on this collection, and I think better of it now than when I played it before.
4. While it is not a 100% positive change for reasons already mentioned, I do like how smooth the special stages appear.
And on that very specific note: I hate the Sonic the Hedgehog 2 Special Stages. They are awful, and I do not care who knows it. I "like" that they are now smoother and wider, but absolutely nothing has been done to improve their "draw distance" around corners and turns so you can see bombs/rings easier and complete the later stages without a map/cat like reflexes. Sure, make me use up a dozen coins to repeat the stage because Tails rammed square into another cluster of explosives right at the goal post. That sounds like a great compromise.
But, hey, I did mainline the game for, like four continuous hours, so I am definitely enjoying the experience.