I loved Sonic 2 back in the day, and realize there were plenty of timing and environmental based puzzles as well as other things to change up the pacing, but at a point it all just becomes rote. You might be doing more then just going right and pressing jump, but after a while it certainly feels like thats all you're doing. If the Half-Life episodes were just more levels with the same stuff from Half-Life 2 people would say the same kind of thing about that series, but with each episode they found new and unique twists on the same core mechanics to make you feel like you're constantly doing something you haven't done before. Otherwise it'd just feel like you were going forward and shooting more dudes. I haven't played Sonic 4 so I don't know what they do if anything to make it feel like something you already haven't done before in a Sonic game. Maybe you can tell me?