I'll admit, some games (specifically SA1, SA2, and the Sonic advance) were good but now most of the new ones are bad because they always have at least two of the below:
-They introduce a character with some horrible monologue that always ends up relating to Sonic (when hey should just bring back some previously shown character)
-If by any chance they do bring back some old character, he/she usually looks and acts different than he/she acted in the other games he/she was in
-They introduce a new transformation for Sonic
-I'm probably going to be hated for this one but Shadow. He may be the worst character in the franchise, he seems to become cornier and cornier every game. They try to make him seem all dark and mysterious or something, but they don't do a real good job at it
-The bosses are weak, but then the bosses after level/act/zone four are hard as freak
-The game might have been made in a short time so it looks unfinished or half assed