I saw an article or blog post about the level design in Sonic 3 a few years ago which went into quite a bit of detail for why it's not nearly as good as Sonic 2, but I can never quite seem to find it. Suffice it to say, in games which are first and foremost about running really fast, there was no shortage of game mechanics which directly conflicted with that. Not stuff that just stopped you for a second if you screwed up, but things you absolutely had to stop for. Far more than there were in Sonic 2 where you could typically keep moving in one direction or another just about all of the time. It was also just a confused mess of superfluous game mechanics in general though.