The AI director manipulates the horror. While his changing of the sounds is a boon to atmosphere, the director's real strength is in mixing up the allocation of bad guys (and equipment). This means that each time you play through a level, it's an unpredictable experience. Yes - there are going to be themes throughout (mock-naively noting "Oh man! Look out! There's more zombies here!" is one of the standard things to say), but in terms of what comes when, it mixes up. You really get a sense of a malevolent personality behind the attacks, which does much to increase the tension and zombie-horror atmosphere.
For example, the more you slow down and take your time, the more likely the director just gets annoyed with you and spawns another wave of infected as a message - though never in sight, in my experience, and always from a logical position. If something can go wrong, the director will try his hardest to make it go wrong - and note that I'm giving him a gender, because I can just picture the bast. As you amp the difficulty up to expert, you learn to expect open cruelty like one of the almost-indestructible Tanks arriving just when you're on your last legs or the Witch - She Who Must Not Be Disturbed - sitting right by the exit, quietly weeping. Which inevitably results in you quietly weeping too.
But the real wickedness comes to light in the versus mode, where you team up with the director and play leading actors in his horrific drama. This is the co-op plus - a totally asymmetrical experience that I suspect will dominate online play. One team plays the survivors. The other plays the infected. In the normal game you get to see the Boomers (enormous fatties whose fluids, if spread on the survivors, will cause a new wave of normal-infected to home in on them), Hunters (prowling, leaping creatures who knock survivors off their feet and slash at them) and Smokers (whose prehensile tongues can grab people at distance and drag them into the shadows). In versus, you get to be them, and work out brutally Machiavellian ways to make the survivors' day as bad as possible. When one team gets to the next safehouse (or, just as likely, is wiped out), everyone swaps and plays the other side. You score points by how well you did, and by the end of the campaign a winner is found. It's deeply compelling. And occasionally you get to be the Tank.
It's almost total win. Valve's new matchmaking system - where you avoid a server browser and just start games - has a few teething problems. PC owners used to more control tend to feel aggrieved when they consider a substandard server and, when something goes wrong, it's tricky to work out what's going on. I've had a few problems with the director, such as spawning a Tank in an inappropriate place - but considering how much it does, that's only been terribly rare.
It's also a game which is incredibly certain what it is. It doesn't make any apologies for its extreme take on the genre. Like Serious Sam before it, it's not afraid of a less-is-more approach to design and a more-is-more approach to enemies. If you believe shooters should be slow and tactical and zombie horror should be Resident Evil, you're going to be alienated. Oh, and while this isn't anything to 360 owners, those PC gamers who haven't gone voice-chat yet need to get with the times. Not being able to yelp at your friends will get you torn apart by the undead pretty sharpish.
Obviously enough, it's worth stressing there's not much here for the single player. But I think the associate oft-stated standard caveat - "as a teamwork game, it's no fun playing with random people" - is far less true than you'd expect. In fact, throwing a group of strangers in a strange situation and then one of them acting strange is the kind of random chaos that Left 4 Dead thrives upon. Things going wrong is glorious. And when things go gloriously wrong, it's the sort of thing you happily blow 500-word intros on. Left 4 Dead is the zombie apocalypse trapped safely inside your computer. Take a holiday in their misery.
In short, Left 4 Dead is another deeply professional, personality-filled and progressive take on the shooter from Valve. In a cultural landscape that has as many zombie touchstones as ours, it takes something special to make them shamble appealingly, and Left 4 Dead, both literally and metaphorically, makes them run. In this new co-op landscape, it'll be interesting to see whether anyone can catch up.