Was reading some info on Alien Swarm, when I came across this:
With that in mind, we've added "Onslaught" mode, which introduces an AI Director that dynamically generates swarms of aliens based on several factors, such as the squad's stress level. Onslaught works with any difficulty setting and ensures no mission plays the same way twice.
Sounds like it's trying to imitate L4D, which isn't a bad thing if I am honest. If anything it seems like a good idea for a game like this. After all, it can get boring killing zombies now and again, so why not go kill some aliens
They've also added some new difficulty levels as well. Just in case the game wasn't hard enough for you
Heaven Doesn't Want Me. And Hell Is Afraid I'll Take Over