Sonic became too complicated. I was fine with side-scrolling through levels, looking for rings and trying to get those special checkpoints for some Chaos levels. Sonic in 3D (not to be confused with Sonic 3D) serves only to infuriate and confuse the hell out of me. I don't need the camera smurfing my eyeballs with crazy angles after going around loop-de-loops.
It also doesn't help that the stories they decided to give an already great game (even as minor as they were) sucked major ass. All I need is some Eggman holding Chao's captive in a very simple canister and I'm set. They should have kept the stories to the comics and tv shows. Sonic SatAM smurfing rules btw.
Plus, the music has been pretty freaking bad. "Rolling around at the speed of sound", are you kidding me!? I wanted to take a bat to my PS2! That song is the very reason I refuse to touch any Sonic title that stretches beyond the Genesis (not really the only reason but it's fun to blame it as the sole reason).
I actually consider myself to be a pretty big Sonic fan despite all the bitching I just did. When I found the Sonic Mega Collection I almost did a backflip, a lot of the titles on it I'd never heard of but then I quickly realized why. They're fun in their own right but not as good as the first three, and then Sega just seemed to quit trying altogether.