Gameplay:
The best 3D Sonic game is Sonic Adventure 2...er, the Sonic/Shadow portions. Those were pretty much about pure speed, but no real path navigation. Sadly, to reach the next Sonic/Shadow stage you had to put up with lots of crap.
Instead of realizing, "hey, maybe we should keep making speed sections!" Sonic Team moves further and further away, adding "team" mechanics (Sonic Heroes), moral decisions (Shadow the Hedgehog) or...I don't what the heck Sonic the Hedgehog 2006 was about.
The GBA and DS Sonic games are 2D and still have that sense of speed and multiple paths that pulled Sonic above the cries of "Mario Clone!" back on the Genesis/Mega Drive. But those aren't the main Sonic games, and we keep getting more 3D garbage.
Character:
Sonic, back in the 90s, was the "cool" character. He was the 'dude with the 'tude. In the US Sega aggressively marketed him as the hip response to Mario. Jumping on turles was dumb, BLAST PROCESSING (lol) was awesome to the extreme.
Nowadays with more "mature" and more violently rated characters, Sonic's edgy attitude is considered bland. He can't be XTREME because he's a anthropormorphisized hedgehog and is "too kiddy." He can't be edgy anymore because he'd scare off the kids. And he can't be the last champion of platformers because, well, Mario's the only real survivor from the 2D -> 3D platformer nuke.
Killing blow:
Sonic the Hedgehog 2006 was supposed to "rescue the franchise" while Sonic & the Secret Rings was a side project. Sega has done serious harm to their brand and will need to completely change their design philosophy if they want oldskoolers to respect the Blue Blur again.
There is no such thing as a console war. This is the first step to game design.
