Next week a free DLC for Left 4 Dead
is coming and one of the things included will be a Survival Mode. Now, you might think that spawning waves and waves is all that entails in the new mode but it's more complicated than that. The designers had to figure a way to ramp up the difficulty so that not all teams hit the wall at the same time and to have great teams separate themselves from the pack by being more coordinated. The Left 4 Dead Blog details
some of their design decisions as well as a chart on what the spawn times for each specific zombie type are determined as you survive the mode longer. What sounds really interesting is the hospital level where the entire area is locked down and you can only reach other areas with supplies as zombies bust through them. The key is to move to each new area as they open up to get more grenades, Molotovs, pills, and health kits. Sounds like it's going to be a blast!