It's interesting, because every level borrows heavily from previous 2D Sonic levels.
Splash Hill Zone (Green Hill Zone)
Casino Street Zone (Casino Night Zone)
Lost Labyrinth Zone (Mix of Hydropolis Zone, Labyrinth Zone, and Aquatic Ruin Zone. Easily the most diverse.)
Mad Gear Zone (Metropolis Zone through and through lol)
Physics take a while to get used to, since they're directional based and not momentum based (I've died plenty of times because I stopped pressing forward in the air, and then just stopped progressing and fell to my death haha.)
Music is... pretty mediocre. Probably my biggest disappointment with the game. 
Lives out the wazoo! (Especially in Act 2 of Casino Street Zone, I went from 10 to 19 lives in that level haha)
But those lives are needed near the end of the game. Has anyone else noticed that near the end of each act, there's always a tough platforming section that leaves no room for mistakes? I've sped through an entire level (with over 200 rings) with little trouble, only to reach the final stretch and die because of some crazy platforming element. 
Like the Zones, the Robotnik battles take heavily from previous Robotnik battles:
Splash Hill Zone (Green Hill Zone)
Casino Street Zone (Exactly like Casino Night, but with less pizazz.)
Lost Labyrinth Zone (Labyrinth Zone to a T)
Mad Gear Zone (Once again, Metropolis Zone)
I have to say, Sonic 4's heavy dependence on the previous Sonic games is both a blessing and a curse. I really appreciate seeing past influences and the nostalgia factor is totally awesome. But I also wanted to see a bit more... newness after 16 years. I appreciated the mine cart rides, the homing attack (kind of one note with the Bubbles badnicks, but it still kept things pretty fresh), and the other new stuff. I'd like to see Sonic Team expand for episode 2 and 3.
...
Lol, all that above stuff really makes it look like I didn't like this game at all... and that is not the case at all haha. Love the game, really having the blast with it. It feels like classic Sonic, and at the same time, still feels very fresh and fun. Bonus levels are pretty tough and unforgiving, but very satisfying.
Yeah, this game is very cool. Out of my 5 star ranking, the game is a solid 4. Good fun!