Is true, but only to a degree, defeat can be funny in bad games a few times (take Jops or raven shield with AI made of distilled gunslinger) but eventually it just gets annoying.
Depends on the style of game too, I'm not sure what happens in the standard L4D co-op mode if a player gets killed, if they can't revive it'll suck, if they get to play as zombie, there's every chance they'll kill themselves to play as zombie. Y'know there's a lot of stuff that's not particularly clear yet.
Along with that, what I've read says 40+ minutes per map, and full FF, I'd imagine the rage is fairly likely to set in if stuff doesn't go well or such. hopefully the AI director will deal with that to a degree, but it can't fix friendly fire and people being bastards.
Success or defeat really has little to do with how much fun 5punkers have, it depends entirely on the game and the amount of alcohol in their system.
Still, a lot remains to be seen for L4D, a lot of details are being kept under wraps so far, hell, we don't even know if it'll use classic server methods, or something new and shiny.