I think the on/off switches are by far the best new feature. There is just so much you can do with that, even without things like switching tracks and conveyors. They are amazing for puzzle levels, platforming levels, puzzle-platforming levels, even troll levels.
Goal conditions are probably the next coolest thing, just because there's so many of them. A lot of stuff that people really had to work at before is now just already implemented by the game. Like the "no touching the ground after you leave the first time" is like an automatic implementation of the no-jumping "captain toad" levels (although I guess that's different because you can't climb vines or things like that either). Someone needs to make a "stealth" level where you dodge enemies without jumping by hiding in the dry bones shell.
Vertical sub-areas will also be cool. People are going to do a lot of really cool stuff with that.
I can't decide whether the dislike button is good or bad, though I think at worst, it's just a necessary evil. The good levels in MM2 are going to be even better, but the bad levels are going to be so much worse. We are going to need a way to sift through them.
Multi-player is weird, because I think the game theory will be totally broken for some levels. Like, you could imagine a "bridge" of on/off blocks, with just one switch at the beginning, and nothing else in the level. Anyone who tries to cross the bridge dies because the other player turns it off, and so no one can do anything. The level is clearly beatable in single-player, but making it competitive makes it impossible, because it's easier to stop everyone from winning than it is to win.
That being said, I fully expect well-designed multi-player levels to be very fun.