Well, I just won New Super Mario Bros. Wii, and I have to say that I loved the game.  I loved the 2-D gameplay and I loved the challenge.   The thing that I really loved about the game was that I played the entire game with my wife, and we both had a blast.   
 
The genius part about this game is the multiplayer component.  If this was a single player only game, I would have enjoyed it but would have ultimately forgotten it.  With the addition of the multiplayer though, it is elevated to a classic game.  The genius of the multiplayer isn't the fact that it's there.  The genius of the multiplayer is the fact that it allow people of different skill groups to play together.  More specifically, it allows people of different generations to play together.  For those that haven't played the game, when in multiplayer, you can stop playing and watch the others play by pressing "A".  This allows you to be a bystander when a really hard part of the game is occurs.  While this may be a turnoff for the hardcore, it wasn't meant for you.  It was meant for your kids who are not quite as good at games (at least not yet).  It was made for your wife who would normally never play games, let alone hard ones.  It was meant for your parents who always watched you play. 
 
By having a co-op multiplayer component that was designed for people of all skill levels to play together, Nintendo has truly made a game for everyone.