I was just talking to a good friend a few days ago about Sonic, and for me, he just hasn't held up well against the test of time.
The series really just hasn't moved much beyond what it was originally IMO. I enjoyed Sonic 1,2, and 3 back on the Genesis, but I never owned them personally. I played them at a freinds house, and although I found them enjoyable, I never saw the need to spend money on them. Outside of a few hidden corners, and nooks, it just didn't give me the feelings of discovery that Mario games do.
Every year or two, a new games comes out in the series that is supposed to jump start the franchise, and every new game is just more of the same. Mario is mainly about running and jumping the same way Sonic is, but Nintendo just makes it fresh with each new game.
I just got Super Mario 3D land yesterday, and although you can say it has some of the best parts from many Mario games thrown in, it is just very new, and very FUN! I just downloaded the new Sonic Generations demo on the 360 a few weeks ago, and I'm like, WTF? This is the same as the last Sonic I played a few years ago. Maybe the game gets deeper at some point, but every Sonic game is just OK to me at this point. The new games look GREAT, and play fine, but they just seem to be fancy versions of 20+ year old games, and sadly for Sonic, his games just weren't that deep to begin with.
Sonic really just appeals to a very dedicated fan base at this point, as I don't know anyone who has bought a game in years. The standard reviews of the last 20+ years shows that. Sega will never have a super high rating(across the board worldwide) for any sonic game until they do more than this running, zipping and loop de loop SHIT! It is just very boring, and not really fun at this point!
Chris