The Hunter is tough to play as, though: it's rare to get a Survivor alone, and your only way of escaping fire is to pull off a spectacular but tricky Prince of Persia-style wall-jumping technique. If you just want to hop into someone's Left 4 Dead game and give them a hard time, try the Boomer. That's the hideous fatty whose primary attack is to blow chunks of semi-digested human flesh on his victim, and then detonate devastatingly in his face with a shower of corrosive bile when shot.
Your victim has three or four awful seconds to realize how totally screwed he is before the Infected get wind of the nutritious vomit. They pour in, in their hundreds, ignoring everyone else and piling on this one sick-blinded, poisoned survivor. As the dead Boomer player, you get to watch in spectator mode as your pretties utterly eviscerate the guy who killed you, and nothing is quite so morbidly gratifying.
With bad ends like that around, you don't have to be told to stick together in Left 4 Dead. But some aspects of playing well as a team are less obvious, and the game teaches you those more directly. If you do find a Hunter eviscerating your friend in time and shoot him off, you get a big green shield icon - a "Defender" merit. Help a near-dead friend to his feet and you're a "Savior" too. Collecting these "Greens" becomes a matter of pride, and everyone's report card appears at the end of each level to show who the best team players were. It means that instead of everyone competing selfishly for the most kills, they're competing selfishly to help each other out, which works out well for everyone.
But nail a Boomer as he waddles towards your teammate and you're likely to get a bright red Boomer-blasting demerit: the resulting explosion does masses of damage. Clipping your friends with your fire, running in front of their fire and using a medkit when someone else badly needs it will all earn you further red marks.
"The most common thing people say to each other after a game is 'What the f*** were you doing?'" admits project manager Erik Johnson. Demerits don't lead to any in-game penalty, but they do go on your Steam Community stats for all to see. And Steam will kick or ban you from the game entirely if it's obvious you're doing it on purpose. After all, there's really no excuse for griefing as a Survivor. That's what playing as the Infected is for.
Left 4 Dead is just one of the games that Valve has scheduled for 2007. To check out the other games click here.