I think this is going to be much better than NSMB2, it has the return of many things not seen since Super Mario World, such as one single, yet large overworld map. the flying squirrel works like the cape from SMW as well... four player option returns which is a plus, and the art style used for the backgrounds of the levels is just great. hope there are good challenges as well.
seeing Bowser Jr. and the Koopalings together is one big plus as well. besides them, I hope they have different bosses like NSMB, such as Petey Piranha, Boom Boom or the Reznors. Ice Mario returns as well, great!
EDIT: seems that Boom Boom is back to the 2D platformers... I feel kinda outdated... -_-
EDIT 2: will there be DLC for this game, like NSMB2? wouldn't mind a few extra challenges, as long as the prices are reasonable...