The level design, and any way to could create a sort of "exploration route" that gives replay value to each of the levels.
And also Coin Rush mode, which basically forces replay and poses a challenge that is actually quite nifty.
I honestly don't really care much for the power-ups, I keep forgetting about them in these games, (I just hope Mini Mushroom wasn't as much of an afterthought as in NSMBW and those ABSOLUTELY POINTLESS mini pipes in some of the NSMB sections. The forcing of Mini Mushroom to access worlds was a stupid forced gimmick, but can't say it didn't make things more interesting). Although, the return of Raccoon Mario does force level design to be more elaborate, and that's good in my book. :I