Jeez guys lay off NJ
Sorry to interrupt the valve-worship circle-jerk, but from a completely objective viewpoint, what they're doing with L4D is a pretty dick move. The game simply hasn't been around long enough for there to be any player-focused sequel. There can't possibly be too much added on, other than new characters, a few new maps and *OMFG CHAINSAW BRZZZEEEEE*. It will be the same gameplay system, same engine because you simply can't develop a completely new game in a year's time. What we're looking at is a stand-alone expansion with a 2 stamped on it instead of a subtitle.
Honestly, in days like this where developers can rework the entire client from patches/hotifxes, and add crazy amounts of stuff via DLC (which they can charge for and no one would care), trying to capitalize off of a full-price release for game everyone's already played is something i have a hard time seeing anyone agree with.
Time to go hide under the table.