What this means is that sometimes you'll be tentatively making your way down a deserted alleyway, while other times you'll be overrun by enemies. Play through the same section a second time and the locations and strengths of each wave of brain-hungry foes will be different. It keeps you on edge, and when playing as one of the infected four you'll be waiting for the humans to be defending themselves from the AI before you attack.
Actually taking control of the infected turns Left 4 Dead into a means of seemingly trying to ruin somebody else's fun. What we played was still a little rough, but the basics are sound. As a Boomer you hide around corners before leaping into the middle of the survivors and belching your juices on them. As a Hunter you wait until you hear somebody reporting the fact that they're reloading before pouncing on them and eating their head. As a Smoker you lock on to humans before launching your tongue at them, and as the Hulk you chuck cars and debris about like LEGOs. It's simple, brutal and in essence quite easy.
Our campaign had us heading for a rooftop helipad across the city, from which we were to radio for a helicopter. Left 4 Dead will ship with four campaigns (with extra campaigns planned for release at a later stage), some of which will take place in rural areas as well as the typical urban settings. Broken into five separate maps, the campaign we played took about 40 minutes to beat. Could that be too much of a time commitment for random online co-op?
It remains to be seen how the ducks will take to this particular water, and whether this will be played more by groups of friends than random strangers, but Left 4 Dead has enough to keep most players captivated for a full campaign. In between maps you're presented with a scoreboard listing your achievements in that round, as well as your failures. Handing over a health pack to a needier player earns an achievement, as do things like rescuing a teammate and exploding a Boomer at a time when it didn't harm anybody. Meanwhile, deliberate pot-shots at friends and selfish use of resources puts a big embarrassing mark next to your name. Serves you right too.