Honestly, I don't mind change as long as they don't pull that furry/human animu bullshit again. That was creepy. I don't even mind a few newer forms of gameplay in the game. Gamma's missions in SA1 were a lot fun while being unique to the other characters' gameplay. The only problems that have been glaring in the most recent Sonic releases I've seen are:
Characters: My main beef in this area deals with Silver, that princess or whatever, and the Chaotix characters. Silver just felt like an elementary school student thought him up. It's like they just went "Yeah, so we have a blue one, a black one... I got it white. Make it happen." There's no imagination involved, it just seemed like a broken logical thing. Also, the Chaotix characters look nothing similar in art style to the other established characters, and they feel outta place. Seriously, why is a bee like five feet tall?
Voice Acting: yeah, they're small animals, but that doesn't need to be reflected by the tone of the voices. They had decent-sounding voices in SA1 (Though the acting was bad), why not try to just keep the good-sounding voices with a little bit better localization?
Gimmicky crap: I think I would have enjoyed Shadow the Hedgehog if he didn't have to carry a gun and ride a motorcycle, and I could probably stomach Silver's levels, if I didn't have to stop every 15 seconds to do some obnoxious psychic puzzle. It's always been the weird gimmicks that have held back Sonic games, such as Big the Cat's fishing levels, and the Knuckles/Rouge levels of SA2. Gamma's missions in SA1 were fun because they added a shoot 'em up element to the established Sonic selling point of speed and platforming.
From what I've read though, I'm looking forward to this game. Even with its faults, the recent Sonic game was decent, and I'm too much of a fanboy to give up on the series now.