the problem is that the sonic games have never really had anything decent to begin with. the only memorable sonic game i remember were "sonic the hedgehog" for the master system and "sonic 3D" which i played on the PC.
the point of the former game wasn't to just run extremely fast through whatever loops and hoops that may have been in a stage; no, it was a genuine platformer. you fell into pits you had to get out of, you were running around in a jungle riddled with difficult ways to cross and you got into a flooded labyrinthine temple, where you had the added constraint of fighting for your life as you passed through under water.
the classic sonic was all about jump puzzles and timing; it had challenges that would make you want to play it (i still hold the original sonic's end boss as one of the most difficult bosses in any game) because there was a high risk associated with the game. you'd die a lot, but you'd feel like a champion when you managed to complete a challenge. even with the switch to 3D in sonic 3D, the team managed to maintain the proper challenging elements. this just goes to prove that 3D sonic is perfectly doable; the problem is that the developers have just been caught up in making the games too shallow. read below:
the latest of the sonic games i've had the mild displeasure of getting my hands on was sonic rush for the DS. the problem with this game was that it became extremely boring and tedious after a while. it felt like work, not like entertainment. instead of having a game challenge you, it was all about speed and how you could beat the level in the fastest time. time trial is not proper game play; it should be used only as an additional feature for hard levels where you want to have bragging rights. this, in my experience, was not the goal of the sonic games.
as an aside, the boss segments in between the various worlds, were actually entertaining. they featured a shadow of the colossus sort of mechanic, allowing sonic to battle various constructs by jumping on top of them - all in 3D. it made me wish they'd make a sonic game just with boss battles.