Valve's new employee handbook has been posted to the interwebs and I can see some glints of
why HL3 has taken so damn long to see light (hint, it has something to do with creating 10,000 hats).
http://cdn.flamehaus.com/Valve_Handbook_LowRes.pdfTL;DR - Valve has no structure on purpose, so people just work on whatever they feel like whenever they feel like.