Ok, now I'm actually pretty excited about NSMB2. And I don't even own a 3DS.
I'm reading through Iwata Asks and I was amazed by just 1 aspect. It's about the camera in the 2player coop. They can't zoom out the screen like they did with the Wii, so they didn't know how to solve it.
Then they came up with a solution that itself turned into a great element: the camera focusses on whoever is the 'leader'. And you can compete for being the leader (by pounding the other player). Watch the video on this page and you'll know what I mean:
http://iwataasks.nintendo.com/interv.../3ds/nsmb2/0/4
Genius. They grab a problem and turn it into a great game element.