Well, shockingly, Left 4 Dead was rated "M" by the ESRB for PC and Xbox 360. Apparently, the projectile vomiting, exploding corpses, undead prehensile tongues, disembowelings, decapitations, blood, gore, and general mayhem warranted the harsh rating.
Honestly, "Blood and Gore, Intense Violence, Language"? The ESRB can be so moralistic and exclusive - this is a perfect game for kids. It teaches them how to survive a zombie apocalypse. Letting them play is actually a civic duty.
Are you looking forward to this sweet, casual education game?
[The British Board of Film Classification has given Left 4 Dead a rating of 18.]