It comes down to this: Left 4 Dead is a hell of a lot of fun to play. The eerie calm right before a stampede of bodies comes chasing down a vacant street is amazing, and there's a palpable sense of teamwork in the air. The little touches are what makes this game so memorable and worthy of survival tales-being able to shoot even when incapacitated, zombies being able to break through doors eventually, the pitch-black office buildings lit only by the torch of your flashlight.
If you don't watch your friends' backs, they're going to die. If you wander off to explore and search for molotovs and ammo, you're going to die. You constantly need to pay attention to the location of your three buddies. If you don't fight together, you'll die alone, bleeding slowly, screaming for help.