Nearly all of the key developers that created the original Sonic games left Sega a while back. Sonic Team is mainly comprised of new people that no one has heard of.
And now that you mention Sonic Aventure, anyone bother to download the port that appeared on PSN and XBLA a few weeks back? What the smurf was that about. Possibly the glitchiest piece ofI have played in a long while. But seeing as Sonic Adventure is one of my all time favourite games, even in the bad state it was in I still had a lot of fun. And say what you want about that game, the music is still awesome.
And speaking of music, there is one track in Sonic 4 I did like. Ta-Da!