Sonic Team boss Takashi Iizuka has stated that Sonic the Hedgehog's older fans are a tough audience, and that trying to keep them happy while providing gameplay innovations has always been a problem. He's hoping Sonic Generations will fix that, though.
"Our team are always trying to present new gameplay innovations so it’s hard to please fans who like the gameplay from the older games," he told ONM. "However, we have included the older gameplay as part of Sonic’s 20th Anniversary, so we think the fans of the older games will enjoy it [Generations] as well. We are also looking into continuing the Sonic 4 series which was released on WiiWare, and we’ll keep developing titles so more fans will enjoy the games."
While I think some of the hardcore fans out there are impossible to please (the kind of psychos who do really long Youtube videos comparing Sonic 2 and Sonic 4's rolling physics), I don't think it's too tough to satisfy the majority of older fans. Just make a game that, y'know, doesn't completely suck.
Mind you, for Sonic Team, that might be the big challenge.