Gabe,
Thank you for this really interesting post. Just a little bit earlier, I was in fact discussing with one of my friend about the fact that Left4Dead was such a great game because you remember "stories" and want to share them after playing it (how we got passed the witch, how we barely made it to the helicopter, etc.)
So, for one thing, the Director works great in that respect. Now for the question: I wondered if the Director's behavior is once and for all set or if it will evolve based on the statistics you gather centrally through Steam. I mean, wouldn't it be great to have some kind of evolving AI based on players' actual response to the game ? A lot of comments made on the game so far on the forums are about the difficulty levels and the fact that there could a level between advanced and expert. This could be solved if the Director's behavior was made progressively more aggressive when players improve (which can be tracked statistically).
Anyway, I truly love Left4Dead for its very innovative approach towards multiplayer, cooperation and storytelling. Thanks Gabe and Valve !