You know what I find funny thinking about Sonic in 3D? How is it that other SEGA franchises & developed games (Jet Set Radio / Jet Set Radio Future, Sonic R,
F-Zero GX*) and a few other games (Trackmania, Mirror's Edge &
Super Mario Galaxy* for example) have managed to incorporate elements of what made Sonic so great in 2D and left a positive mark on many gamers minds? The truth is that it isn't engines, limitations or anything technical really. It's all about Sonic Team being too lazy to try and be more creative when it comes to truly translating Sonic into 3D. Things look like they are shifting for the better (not necessarily the best) though with the new Unleashed team and the Hedgehog Engine, but when you hear things like "Sonic is all about speed" only to hear later that games can't fully consist of that sense of speed because of the limitations involving Sonic moving so fast, you have to wonder why no one's re-evaluating things as a whole with the franchise.

Also fun fact: Sonic Unleashed shares a lot more in common with Sonic R & the Saturn 3D Blast special stage design due to the fact that the programmers in charge of both of those games worked with Yoshihisa Hashimoto (big guy behind Unleashed and a programmer for elements of the Sonic Adventure games) to create the game.
*Italicized for delicious Nintendo does what SEGA didn't irony.
Anyone who thinks I don't have a part of myself reserved for SEGA, I bequeath to them, a boot to the head.Also, I demand that everyone here play through the demo for
Sonic eXtended Genesis (XG) just because it's relevant to any Sonic fan's interests. While you do his, compare the amount of ideas there to some of those found in recent games. You can be sure that the difference in caliber is nothing short of "wow"-inducing. This isn't to compare 2D to 3D by the way, it's to show he vast difference in ideas and direction that the newer games and older style have.