Nintedo's complete failure at designing online play.
Seriously, anyone who's played Brawl knows what I'm talking about. Brawl is my favorite multiplayer game to date (BTW, I'm not even going to mention tripping here. We all hate it, we all agree on it, end of discussion.), but online matches are stuck on timed, 4-player fights, which to me is the worst way to play the game. I've never even been able to play a match without game-ruining lag. I hear they've somewhat fixed it, but I don't care as I don't even want to play a timed match anyway.
I'm sure everyone already knows how much this sucks, but I have to mention it here because this actually kept me from buying a Wii. My Fiancee already has a Wii, so I can just play the single-player games I like at her place. My friends all have Brawl already, so the only reason for me to get a Wii would have been to play Brawl whenever I want.
Mario Kart is much better, but again, you can't pick what mode you want to play in. I was really excited about the strategic setting, since it allows you to have a more legitimate test of skill rather than the usual craziness. It's still fun to play as it is, but I would have been able to take the game a little more seriously if it could be played with strategic items only and it would be another reason to get a Wii.
I know it's a little strange to be pissed about Nintendo sucking at multiplayer when the other systems and PCs have a far greater selection of games anyway. The thing is, I'm completely bored of the FPS genre (I hear Shadowrun at least made an attempt to be more interesting than most, so I may give that a try) and I'm terrible at RTS games (though I still play WC3 for the custom games), and I absolutely refuse to play MMOs. Since these genres make up 99% of the online multiplayer games available, I think it's understandable that I was looking forward to Brawl as one of the few online games I could really enjoy, and that I was quite disappointed at how hard they failed at it.