Those solutions are so subtle, one might not even realize that's exactly what they are. How do you build a spirit of teamwork? Have players constantly risk everything to save their friends, then frequently force those friends to return the favor. How do you keep players from hoarding power-ups? Reward them for giving them away. How do you break up a long campaign? Let the players occasionally switch teams and play as the enemy. There's a wealth of really fresh thinking on display here.
A good match of Left 4 Dead is a singular experience, one that created its own vocabulary, its own codes of action nearly overnight. When we hear now that a friend or colleague is a "witch shooter," there's no further explanation necessary. (And yes, Griffin, we're looking at you.)
The allure of Left 4 Dead isn't just in its design wizardry though. The motivation of working as a team to bring down a tank works, in part, because of the disturbing audio and visual cues that accompany his arrival. Likewise, dirty, dirty witch shooters like Griffin are all the more vile because the witch's haunting cries make the moments leading up to his treachery so tense.
Through an almost absurdly simple premise, Left 4 Dead addresses some big ideas, providing a road map for future developers wishing to create true co-operative experiences. It may be just a skeleton, but we can't wait to see how other developers flesh it out in the coming years.