I had this problem to. I just kept trying to open it over and over again and eventually it worked. Steam updated earlier this evening so it could be that. When it finally did work, a box came up saying something like "finishing installing demo".
From: DevilousSniper | #006 Go into the game's properties and put "-windowed -noborder" and it won't crash on startup like it did for me.
Did that first.
From: moothead | #007 ^ Please explain your sig :P.
This crash probably has something to do with me not having pre-ordered L4D2, but still preloading the demo and playing that. (by double-clicking L4D2.exe) I'm fairly sure it's just a demo, so it's not illegal... Right?