After complaints that the previous 2D Mario Bros. title on the Wii was embarrassingly easy, series creator Shigeru Miyamoto kicked New Super Mario Bros. Wii up a notch or 3. Instead of the cake walk we had in 2D before, expect a solid challenge with New Super Mario Bros. Wii, even in co-op - maybe the Demo Play feature will come in handy after all.
When asked about the game, Shigeru Miyamoto simply said it's "pretty difficult" and we now know that New Super Mario Bros. Wii celebrates the hardcore fans that grew up with Mario. You and your co-op buddies will need to use all of the "Super Skills" in your arsenal, as well as avoid jumping on each others heads too often, to win this one.
Source: Joystiq.com