Cool beans. I eagerly look forward to easier custom map support and installation.
However, what I really want to see added to Left 4 Dead 2 is the campaign(s) Valve was making for Joss Whedon's movie "The Cabin in the Woods".
Apparently, Whedon and Goddard wanted to do a game-tie-in for the film. However, knowing full well games of that type have always sucked, they wanted to try something different.
Their plan was to approach a company they knew doesn't half-ass their game designs. They considered a few but when they thought of Valve; the company that made Left 4 Dead (a game they liked); they knew they had a fit. More over, they knew how to do the tie-in. An in-theme campaign for Left 4 Dead 2.
It didn't take much to convince Valve to go along with plan. They started immediately working on level designs, character/creature art, voice-overs, etc.
However, production delays and MGM going bankrupt not only threw the film production into limbo, it pretty much killed the campaign tie-in.
You can still see some of the work done for the tie-in within the film. Some of the creature designs for the monsters within the film were done by Valve artists for the campaign.