A few dozen undead corpses in and we come across the first of the four player-controlled infected. In Left 4 Dead, the four humans appear through walls as yellow silhouettes when not directly visible, meaning teammates can always tell where the rest of their team is, even from rooms away. It also means the infected players can ambush the humans simply by waiting around corners.
In this situation, the Smoker has been waiting for us, perched at the edge of a hole in the floor above, looking down into the room we're about to enter. The first clue that there's a Smoker nearby is, well, the smoke. It fills rooms and makes it difficult to notice a long black tongue snake downward and wrap around your teammate's neck before lifting him 4ft. off the floor. Once he's trapped in the fleshy noose, he'll uselessly dangle there, with only a few precious seconds of life left. It takes the quick-thinking trigger finger of another teammate to persuade the Smoker to let go. Score one point for the survivors.
Whether they're trapped under a Hunter, lying wounded on the floor or just perilously clinging to a ledge after being punched out of a window by a Hulk, saving a teammate is an awesome feeling, and it creates countless unscripted moments of fear (and subsequent relief). Small touches like the ability to lock doors as well as shoot holes in them add some spice to the proceedings, while the so-called AI Director works silently in the background to orchestrate the flow of zombie hordes.