The design, how the friends and enemies are (Mario has friends, but done in a right way), how the 3D (and new 2D) games play, how bugfest some of them are and Iizuka.
I have Sonic generations, I like Sonic Generations, but I think is starting to have too much praise, when its not that good to begin with, it has problems that people seem to overlook because the old ones were so horrible. Dont get me wrong, its a good start if the franchise started to go up from here (as I said, even if its not excellent, its a good game, with good ideas), sadly with Iizuka im pretty sure they will get wrong again what they got right with Generations and start adding more trash, than will only hurt the game as a mascot platformer.
Im sorry but the only thing will save the franchise (and im talking about creating platformer amazing games like Mario, not continuing to do, only good games, that is also a possiblity) is kick out Iizuka and give it to EAD to make a FULL reboot. Designs, friends, gameplay, absolutley all needs to change.
Dont worry about kids sales, they will buy it at the end, even if the designs turn more oldschool.The good thing is that when they are older they will see that they were playing not only something they thought it was cool, but also a game that was excellent.
Too bad this is never going to happen.