L4D you dont need to teach people anything really. You can play something like Payday, and you have to tell your friends ok, so we need to go in through here, hack this camera, then drill this etc etc. With so few co-op games existing nowadays - especially so few that allow 4 player co-op, it's always going to have staying power for that reason, and the gameplay is also incredbily straightforward to pickup. There's no grind, or commitment, it's just a game you can throw on with some friends and play, then leave and not get behind on things Ironically i would argue that the reasons you listed for it not having longevity - no unlocks, skills, abilities is actually the reason it lasted so long.
