Actually, mik's got the major things. People are generally dicks anyway on multiplayer games, but it doesn't really hurt anything, at worst, it's a bit annoying- things like assholes who disconnect when they're clearly losing. In MM games, which generally rely heavily on alliances and a lot of cooperation, they can do much more damage. Aside from that, in many, the continuity of the thing, combined with the "experience" system which seems so popular, means that even if everyone else plays fair, some Cletus and half-dozen his redneck pals can bust in, prey on the newbies, rack up massive amounts of power, and basically kill everyone in sight. That's the sort of thing that ****ed up UT.
Never mind that they're usually buggy in the first place. Anyone remember Anarchy Online?