I am still looking forward to this game , because NSMB on the Wii was amazing and I loved every minute of it. It sounds like this has a dedicated single player mode, so for me, this will be a great 2D single player Mario game. However, its very unlikely I'll get many chances for me and my friends to all meet and play its multiplayer mode and the lack of online option for it is pretty disapointing. Like MCJ said, its pretty much a given for 360/PS3 games and I don't see why should be any different for the Wii.
if the the game was designed for multiplayer, I don't see what would have to be changed if this game had an online mode.
The only issue is communication between players, which they could use Wii speak for.