Besides lacking opportunities to barricade a door against a crowd of zombies or rip through them with a chain saw, and the notable absence of a shopping mall campaign (too cliche, Valve?), Left 4 Dead delivers every moment you could hope for in a zombie game. Case in point: you get to legitimately shout to your teammates, "Leave me, I'm done for! Save yourselves - GO!" while you try to buy them some time by firing your dual pistols into the descending horde as you lie wounded on the ground. And as awesome as that is, the unique experience of playing as the boss infected almost steals the show from the survivors.
Left 4 Dead's emphasis on teamwork and vague hints of story may not appeal to gamers who demand a focused single-player experience, but for the millions out there who have ever imagined their role in the inevitable Zombie Apocalypse, it simply cannot be missed.