Mario is an extremely precision based game. In a world 7 castle there's a pipe under a platform that takes a perfect spin jump to reach. If you lag there you're dead. The game was designed so that perfect jump got you there. To some extent that's how all Mario games play. NSMB Wii is even worse with lag because of the immaculate level design. You can tell there's a speed run hidden in most levels, even more so than the old games. What's lag do if you have to time the jump on the koopa perfectly?
Lag in Mario is worse than other games. You can't die as easily in COD or other FPS games. We have spray and pray and lag leading there. There are so many bullets that you have a probability of hitting and they don't usually kill you instantly. In Mario, you miss a jump and you're replaying half the level.
Even LBP has gameplay differences that make it possible for multiplayer. The precision in jumping and timing is nothing like Mario. It feels much sloppier and less reactive with jumps, where as in Mario I know exactly how long and hard to press the button for the jump I need. Additionally there are many respawn points in LBP right before hard areas, each with several lives. In NSMB Wii, you lag, you die. And start over from the beginning or halfway point. That is a fundamental change in the gameplay. Including checkpoints like LBP would have made the game a joke.
If you guys can't find friends to play Mario with or want to play this game so often that you can't play something else at 3am, you guys are more Mario obsessed than myself and my friends. And if there's one thing I learned from SSBB it's that online multiplayer sucks in games where the main joy results from yelling at and laughing with your friends.
I didn't even mention how lag would hurt 4 players. Sometimes you're all on a small platform and one player is bouncing off everyone's heads for several seconds trying to fall in the only open spot. Can you imagine that with lag?
If people think lag wouldn't affect this game they never really understood Mario. You can walk or move safely through most levels and die far less often, but that's not how people play Mario. The point is to run as fast as you can and bounce off everything and anything in a frantic race to the end, perfectly timed. It's like people who stopped running in Sonic. You weren't supposed to stop! Can you imagine lag in carnival night zone?