http://store.steampowered.com/app/552500/Warhammer_Vermintide_2 I've been addicted to this game. I heard that it was pretty much the spiritual successor to L4D2 so I automatically grabbed it along with 2 of my friends. It really holds true. The only thing is that it's set in the world of Warhammer. The game is primarily melee focused with some exceptions. As far as similar things to L4D, vermintide has an AI director. You wander around for too long, the director will trigger hordes to come after you. Someone wanders away from the group, it will trigger a special mob and it will pin them down. If you get pounded until no more HP, you'll be in DBNO state and will need reviving. You fall off a ledge, someone will have to help you up. If you die, your team will have to progress further to free you (respawn). You can play with other people through matchmaking, private games with friends, or with bots. The game has 5 heroes to choose from and they have 3 different careers paths as you level up. There's also a talent system as you level up. You get new equipment through loot boxes at the end of each map. You can upgrade the loot boxes by finding hidden items in the map. You also get loot boxes as you level up. There's a crafting system as well.
I remember playing L4D, it was a unique concept back then. Had to look it up, that was back in 2008....