I do love co-op games, but the ever-amusing Angry Video Game Nerd got a short play at E3 and he says it is more of an ass hole game than a co-op, as you do things like interrupt each others' jumps.
(which he said he loves)
EDIT: His full quote:
NEW SUPER MARIO BROS Wii – Of all the games I played at E3, this is the one that I had the most fun with. It’s in the style of “New Super Mario Bros†on DS, except it’s an all new game. It resembles Smash Bros from afar. But it’s very different. It’s simply a sidescrolling Mario game with 4 players at the same time. This means you all compete for coins and powerups and be a total dickhead toward’s the other players. It’s similar to the original “Mario Bros†arcade how you can get the other player killed, but just amplify that a million times. Say for example, the other player is trying to jump a pitfall. You can jump on top of their head and make them fall. It’s an ******* game! And that makes it fun. They should just call it "Super Mario Bros: *******".