Branching this off from the Wii Sales Thread.
In that thread, we wandered off the path and started discussing the definition of a "party game". My contention was that the Wii library leaned towards "party games", in that even the Wii (Nintendo) interpretation of traditional genres (fighters, racers, etc.) leaned towards the "party" side of things. But what makes up a "party game", exactly?
I'd say that it's a combination of 1) accessible, easily-learned controls (that may or may not have considerable depth), 2) accessible aesthetics (i.e. no space marines or Herculean alien-killers here), 3) 4-player/local multiplayer are front-and-center in the game's main design.
Some have wondered what, in my mind, doesn't make something like "Halo 3" a party game, since it supports 4-player. I thought about that, and I'd say it's the fact that the single-player game is also a large part of the game package in Halo 3. In Brawl and Mario Kart Wii, the one-player game is a sort of afterthought. Both games are clearly geared to get multiple people in a room, playing together. The "solitary gamer" aspect isn't very prominent in their design.