Usually, the AI is reactive to the player's actions, while the story dictates the order of events. Left 4 Dead switches things around. The AI, based on a new system known as Director, manufactures a unique experience for every player, in real time. If the survivors have been through a nightmarish gun battle, Director stops spawning enemies and holds things off, allowing everyone to catch their breath; if they've been wandering around doing nothing for a while, the AI steps things up, throwing in a bunch of zombie aggressors or even a quick boss battle. These flashpoints can happen anywhere at any time; it all depends on the actions of the players. "Director creates highs and lows, without it being tied to navigation," says Faliszek. "There is an over-arching story, but it doesn't try to explain everything."
Excited about GTA4 and I don't even own a copy. Left 4 Dead (discussed here) sounds like it might be pretty interesting too – I watched M. play on Portal (same makers) which was pretty amusing – esp things like the impossible geography of creating a tunnel in the floor of a room that brings you out into the same room via an entrance in the ceiling.