First off, it has already been said that the difficulty is going to be ramped up for this one compared to the DS one. Second, Wi Fi would just have me playing with kids who think its cool to throw me over a cliff every 5 seconds.
Online co-op would have been nice, but the Wii is about playing games with your friends outside Wifi. It encourages families to play together and have fun, so naturally they are going to keep that going with Mario.