These are the patch notes from Steam.

The Left 4 Dead Survival Pack is now available for the PC and Xbox 360, free of charge to all owners of the game. The full change list for the PC release is below. In addition, both the retail and Steam versions of the game (PC and 360) are now available for a special price. Check Steam and local retailers for pricing details. For more information about L4D, please visit www.l4d.com

Survival Mode


New gameplay Mode

16 maps including Last Stand

Leaderboards

7 new achievements


Versus Changes

2 new versus campaigns

Fixed a case where versus team score would continue to be recalculated after the score panel was displayed

Fixed an issue with recalculating the versus score health bonus for a player when they are rescued from being incapacitated

Fixed being able to start a swing as an infected, then teleport to the survivors and have the claw swing hit a survivor

Fixed a case where teleporting to the survivors would not reset the ability timer

Fixed an exploit where you could load the chamber of a weapon with bullets by holding down the melee key

Fixed a case where it was not possible to join a game while it was in a finale

Fixed a rare case where listen servers would speed up the game over time

Removed convars related to melee fatigue

Fixed a case where infected players would get a respawn time on their initial spawn into a map


Server Changes

Made ms_force_dedicated_server not require cheats

Add a message when queuing a heartbeat

Simplified setting of game mode

Removed convars director_no_human_zombies and director_holdout_mode

Added mp_gamemode [coop,versus,survival]


General

Added lobby join and leave messages
Can't wait to have a game of it later.