Gamemode Logic: Designers can now make a single map that can behave differently depending on the gamemode in which it is being played. This can dramatically reduce the file size of Campaign add-ons, as the same map can be used for Co-op, Versus, Scavenge, and Survival.
Director Logic: Maps can now alter their layout and behavior depending on how "angry" the A.I. director is. The better the Survivors are doing, the angrier the AI Director seems to get. This allows you to base variable paths and other events on the player's skill and current conditions.
Scavenge Mode Support: All the elements map authors need to support Scavenge in their maps. This includes game props as well as a full example in the form of Deadline 2, an updated tutorial campaign.
If you simply want to play custom campaigns instead of make them, you'll need to download the add-on support files through Steam. This will allow you to run user-made campaigns from Left 4 Dead as well.