The general "feel" of the level is off I think, it looks like it should be a "city in the clouds" of sorts, but it feels almost enclosed somehow. I think to start with you should do away with the cliffs, replace with stone/concrete what have you. They look out of place and detract from what you have. The sky is too dark for a "cloud city", look at examples in recent games:
World Of Warcraft: Cataclysm
The Legend of Zelda: Twilight Princess

Both of these follow a very similar visual design, smooth, white, clean. Your map is using the generic HL2 textures that were designed for a dark, Orwellian city in Europe.
What the map needs I think is a lighter sky, something that makes the player feel like they are literally walking on air (they are!) and can hopefully knock the darker, almost dingy textures of HL2 into looking lighter without actually being different textures. Good cubemaps, HDR and a "warmer" fog if that makes any sense.
Good example:
Portfolio piece from one of our own here at Interlopers, forget who, sorry ;p
That is all stock L4D, with a CSS skybox. You just need to learn to use what you have right at your finger-tips right for the style you're going for.
Good luck with the map, you got a good base there but it needs work if you want to get the environment to "that level"
Hope this helped
