Here are the worthy details:
The goals for Survival Mode are "to deliver a mode of play distinct from Campaign or Versus, have games that regularly last under ten minutes, and emphasize competition with team play through leaderboards."
The Hospital Elevator portion of the 'No Mercy' campaign has been revealed as one of the survival mode areas.
The mode adds a 'lull' variable which is simply a brief period of time where nothing new will spawn, the 'lull' will not be highlighted by any signal, it will just happen and the survivors will have to react in time.
It is much more complex than a typical survival mode that simply increases the volume of spawns as the players survive longer. Rather each form of infected (Horde, Specials, Tank) have individually set spawn times that adjust separately from each other, so for instance in the first couple of minutes the nearly impossible combo of three hunters and a tank could appear at the same time; likewise, several minutes in things could seem easier than when the round opened.
Ammo piles have been moved to less defensible positions on the maps. Which means making an ammo run is "rarely a safe proposition and requires good teamwork and planning."
