Because there are so few co-op games available nowadays - especially those that allow for 4 player co-op - it will always have staying power, and the gameplay is also really simple to pick up. Evidently, the player base of L4D2 is proliferating as time progresses. You don't actually need to teach folks anything in L4D2. It's only natural to put a gun in someone's hand and have zombies charge at them to be shot.

Because there are so few co-op games available nowadays - especially those that allow for 4 player co-op - it will always have staying power, and the gameplay is also really simple to pick up. Evidently, the player base of L4D2 is proliferating as time progresses. You don't actually need to teach folks anything in L4D2. It's only natural to put a gun in someone's hand and have zombies charge at them to be shot.