The game looks like a graphically sharper version of New Super Mario Bros. on DS, it has the same visual style and effects. Items like mushrooms float around in bubbles, and when you lose a life while playing cooperatively, you come back on screen in a bubble. After losing a life the demonstrator told me that I could shake the Wii remote to bring my bubble closer to the other players. Another player has to touch the bubble in order to release you back into play.
It seems like their designing the same sort of hidden areas into New Super Mario Bros. Wii as they did before. The large coins also made a return in the level I tried. The first player took the role of Mario, I played Luigi in the second slot, and the third player took control of toad. I couldn't see who would fill the fourth role.