Before we even go into what the additions are, let me point out that this is only planned for the PC version of Left 4 Dead; sorry, I hit the character limit in the headline, and didn’t mean to get anyone’s hopes up.
The Left 4 Dead Blog informs us that sometime after the Survival Pack (coming in a few weeks) is released, Valve will be tweaking the matchmaking system to include third-party campaigns. This means that you will be able to find dedicated servers for any custom campaigns you have installed, and also see which of your friends are playing third-party campaigns.
On top of this, more features are planned for release later on, the most exciting of which is team-based Versus matching. In other words, if your team of four has a specific team you’d like to face off against, you’ll be able to locate each other now.
Lastly, the way in which players are matched will eventually be tweaked, although no specifics on how were given. *Insert comment here about how Valve fully supports PC but leaves Xbox 360 players in the ditch*
