The Left 4 Dead blog has an interesting little piece about how they designed the beautiful images of the game.

As far as I’m concerned, the game would be much less awesome if it didn’t look as nice as it does.  The gameplay is excellent, sure, but it’s all the time where you are waiting to be attacked that needs the benefit of the art direction mentioned in this post.  

Without compelling scenery I feel the moments of downtime would run the risk of appearing drab, but thankfully they look outstanding.