My Issues with Sonic 4
Some of this is nitpicking, I'll admit.
Physics
Sonic no longer retains any momentum on his own. Rolling down a hill brings you to a gradual stop whereas in Sonic 1-3 it would cause you to build up speed.
He also moves rather floatily when in the air. A simple jump allows you to 'waggle' about in the air if you alternate pressing left & right a few times. I'm not saying that it should be realistic, but it doesn't make any sense. Influencing movement in the air is one thing, but being in near-complete control is dumb.
Spindash is slower than running. Guaranteed, if you spindash you will not make it up a steep ramp. If you just walk up it though, you'll build enough speed to do it. This is pretty much the complete opposite of Sonic 1-3.
Sound
While the first stage's music is catchy, the rest of the music is flat and bland. It's really hard to believe that these tracks were composed by Jun Senoue. And what is with that one synth instrument that is overused?
Sound effects for spindash and roll are switched. I'm sure this has been incorrect since Sonic Rush and I'm not sure why they continue to make the same mistake. It's fairly obvious if you've ever played a Sonic game before.
Worst Boss Theme ever.
Level Select
I just don't understand why this is available after the first act, it's not even an unlockable feature. Understandable in, say, MegaMan, but when progression through increasingly difficult levels is what the game's all about it just doesn't make sense and isn't appropriate.
Homing Spin
To be honest, homing spin doesn't really bother me. What does bother me is the ridiculous amount of homing spin-based sections per level. Having a new game mechanic is one thing, but overusing it is a terrible idea.
AutoPilot
Okay, games that play themselves look cool but when you actually want to play the game yourself it's goddamn annoying. Those sections in Death Egg Zone where you were guided by rings of light were the worst part of the level because they basically had you sitting there watching for about 10 seconds. Sonic 4 has sections like this in EVERY level. It's like it just wants you to sit back and do nothing - not even hold Right in some cases. Admittedly, this has been a problem with all Dimps' Sonic games.