Out of what will likely be branded stupidity, I decided to wipe out my hard drive and install Windows 8 (not pirated, available through technet). Everything installed fine; restored all my games from backup with no issue. Go to play L4D2 and it loads alright, though it doesn't pull down my settings from Steam Cloud. Whatever, I'll just go through and change the resolution back and eventually have to rebind all my keys.
I change the resolution, then the game crashes. I try to open it back up, crashes within 5 seconds. I've tried deleting and redownloading, but it doesn't work.
What's odd is that it does work if run as admin from the .exe in the Steam folder; however, it then tells you you're using the -insecure option, and will be booted from VAC games.
So a fair warning: if you were/are planning on upgrading to Windows 8, probably best to hold out until it's verfied that L4D2 works on it. Luckily I made an image of my hard drive before nuking, so I may just reload that to be able to play...or use a Win 7 vm I already have set up.
Update: GameTracker was causing the crashes, not the games or the hl2 engine.