Valve finally added a survival mode to Left 4 Dead. Insanely hard if you don't have a good team. That meaning coordinated more than anything. Then with a good team it is just hard if you don't know the level well. Survival mode is closer to what I expected the entire game to be like. Endless horde bum rushing you from start to finish.
I expected the game to be an endless horde rushing you with specials mixed in from time to time. Having to hole up in a location and fight until the horde subsided for a bit allowing you to run to a new location. Or using the tank as a way to scare the horde and hope you can run from the tank long enough to reach some form of safety. OR even just play it the good old rambo style and fight your way through the horde out in the open in hopes of a big pay off.
Survival mode is really just the normal panic and finale events that are found through out the game anyways. Plus a few new ones. Only the worse case scenario where what ever you just did to try and progress back-fired. You are now trapped with an endless horde of zombies bearing down on you. Now you have to try and take as many as possible with you. How long can YOU survive?