Hi all,
For some reason, whenever trying to launch L4D2 from Steam, I'd get an error saying something like "That game is currently unavailable. Please try again later. " Just in case anyone else encounters this problem, I found a solution:
From this forum : http://forums.steampowered.com/forums/showthread.php?t=1035480 , the first reply links to https://support.steampowered.com/kb_article.php?ref=4595-WEXN-6831. You will want to follow step 5 listed there, "Verifying GCF Cache Files". Let that finish, and then try to relaunch L4D2. It did the trick for me.
In summary, basically right-click on L4D2 in Steam, pick Properties, then in the window it brings up, click the Local Files tab and then the Verify Integrity of Game Cache button. Then let finish and and launch L4D2.
Hibba.