the AI director is just a very very complex RNG. if you dump the cvar list from L4D1, nearly everything feeds the system data or grabs from it in some form or another. That said, there really isn't enough room in the ~1.5 sections to see much improvement, BUT spitters LOVE coming out of the woodwork during a boomer attract mob, so there is evidence that some of the changes are for the better.
I'm surprised that no one mentioned how much more optimized the game is now, by the way. Runs smooth at my desktop resolution (1280x1024) on my lower-end video card, while i had to drop down to 1024x768 for the same frames on l4d1... which only really continues to enforce my idea that l4d1 was basically the l4d2 beta.