Okay... something's messed up with the newest (18.104.22.168) update.
I've installed it and then reapplied the re-emu (both the "2054+" and then tried the "rev-emu 2.0 epc edition") both with no luck.
left4dead2.exe shows up in my task manager for about 10 seconds, then disappears. The game never starts.
I then reapply the last update (22.214.171.124), and it starts the game no problem.
WHAT'S UP WITH THAT???
The "sfk" version I got from the tunngle site was the issue.
It's different than the 126.96.36.199 update from here.
Installed the "L4D2_v188.8.131.52_To_v184.108.40.206_Update.exe" and have had NO PROBLEMS.