They compiled it for the latest source engine and updated the shaderz though ... lulz.
The big problem is that ever since Left 4 dead 1 (the last "good" multiplayer game on the source engine IMO), there's been something "off" with the way the hitreg feels and works.
It's a combination of model movement, hitreg, hitboxes, FOV, "feel", weapon models, etc. etc., but since L4D1 which I absolutely loved to death, all the new games including portal 2 and l4d2 have felt like "bricks". Just try it - install l4d1 and 2 side by side, and fire them up, and then play them for ~5 hours each in multiplayer - you'll notice the difference.
I blame consoles. That's also the time when source was ported to xshit360degrees.