People love slaying zombies according to Valve's Chet Faliszek who stated that the series has sold more than 12 million copies.
He was speaking to VG24/7 about Valve's approach towards promoting their games, saying that the company doesn't rely on marketing because "when a game does really well at pre-release, they're going to know that you're not just talking a bunch of PR crap."
We just let the game speak for itself because the internet has made this thing where you can't pull the wool over people's eyes any more. They're going to know you're hyping,"
Sounds like Left 4 Dead simply speaks for itself with those kinds of numbers.
By Stephen Heller