This just occured to me while reading about the forthcoming
Borderlands, which is touted as four player co-op action-adventure.
I don't think it really is possible to do multiplayer co-op unless your compatriots are dedicated to taking it 'seriously', or if the game forces them to.
I say this as I've played a lot of singleplayer-oriented games that have a co-op mode, and have found that playing them is like an episode of MST3k... you skip through all the story bits making sarcastic jibes, laugh your way through dramatic moments and any suspension of disbelief rocks wildly before collapsing like the Tacoma Narrows bridge.
System Shock 2, for example, became little more than a first person shooter, but one with more limited resources. Messages from Polito, audio diaries, coming across the corpses of the distressed or scenes of tragedy... all were scurried through to the next objective.
Halo 3, likewise. For all its po-faced seriousness, the desperate struggle of humanity is laughed at merrily when there's four of you, as you accidentally crash your jeep or announce how you just knocked a Brute flying.
Duke Nukem and Doom, while very light on story (kill the aliens!), similarly become little more than a blur of scenery that you rush past to reach the next level with your cackling cohorts.
Which brings me back to Borderlands. Maybe I've always played with the wrong people, but I can't imagine playing it co-op and getting anywhere near as involved in the story as if I were to play it alone. Conversations with NPCs would be skipped through, plot twists and revelations would be talked over, and the entire time would be little more than a whirlwind tour through it all.
I can't imagine pen and paper roleplayers rushing their campaigns in this way, so why does it happen in video games?