I just saw the new trailer over at IGN and it looks awesome. Im not sure but this is what I have figured out so far. 
-Play as Mario or Luigi 
-Play 2 player co-op (not alternating like in the old days) 
-Mushrooms, Fire Flowers and Stars all did what they use to. 
-able to do some of the 3D Mario moves like jumping off of walls. 
-Use both character to do special moves. (I just noticed Mario jumped off of Luigi's head and it gave him an extra boost, but there might be more) 
-Stages resemble that of Super Mario Bros 1, 3, World, and Yoshi's Island. 
-Circle formation stars return as check points and end of the level flag sliding is back. 
I don't know about any of you but a 2D, simultanius, co-op Mario game has been what I have been dying for since SNES days. The game looks just as fun as the classics but with a bit of a modern look. The only thing it needs is about a jillion and one levels just like every 2D Mario befor it.