He was too goddamn hard to control. That's my frustration with Sonic in the current "console generation". I understand that Sonic is fast, he's named Sonic after all, but in the old games he was controllable, and the thrill came from the dangerous terrain and the unexpected but simple & welcome challenges of the level. Not making him so fast you can't see what the hell you're doing, where you're going, or give yourself those few seconds to prepare for what lays ahead.
I played a bit of Sonic Unleashed and I couldn't get enough of a grip to feel like I was playing something FUN. It was too rushed and poor gameplay mechanics combined with him going "YEAH!" "UGH!" "WHOAAAAAA" every three seconds doesn't serve to do anything but frustrate the living hell out of you. Yes, we understand: Sonic is vibrant and free-spirited, but we got that when he DIDN'T TALK, too.
Unfortunately, the only good Sonic game that's come out recently was Sonic Chronicles by Bioware. They kept the battle system and cast streamlined, and with the plot, kept it simple, kept it true to what Sonic was. Extra unfortunately, the Sonic Team doesn't seem to be hearing that we, as fans of the franchise, don't want a mashup of whatever flew into your head at the time, we want something clean, simple, and if you have to insert your gimmicks with gameplay, make sure they WORK.
I doubt it'll happen anytime soon, though. D: