Firstly of all, there is little wrong with either Sonic Adventure or Sonic Adventure 2. Both were excellent games that pleased most Sonic fans around the time of their release. Sonic Adventure pretty much blew me away when I first played it and remained enjoyable to this day.
Secondly, 2D Sonic games were not "all about speed". So many people make it sound like all you have to do is hold right on the direction pad to play which is fundamentally false. Most of the 2D Sonic games reward exploration, which added a sense of depth to the game.
Thirdly, why is Sonic the only game that needs to "stay true to its roots"? Basically no other platformer has that requirement. Mario is in no way similar to the 2D games and thats not a bad thing, same with Donkey Kong. The skills of Sonic, Tails and Knuckles opens the possibility for incredible 3D level design, there's no reason why they need to stick with the holding forward and occasionally jumping gameplay.
Fourthly, Sonic needs a complete reinvention. It needs to pick and choose what works and what hasn't but essentially start from scratch. Probably the only thing I would recommend keeping from the 3D Sonic's is the Chao artifical life simulator because it essentially creates incentives for incredible replay value. Other than that SEGA needs to go back to the drawing board and think outside the box.
