1. Some of the props doesn't quite fit in or looks a bit odd. For example the "Zutritt Verboten"-door at the cilo-looking thing. There are also a couple of massive concrete road blocks lifted by a forklift on one of the sides of the map. Maybe a smaller prop would fit better, (because I know you had to block it off).
2. You have some very blank spaces in the level that needs details compared to the rest of the map. They are especially in the spawns and outer lanes. You could probably make something good looking by opening it a bit up and making some 3d skybox features. (But that might be hard.)
3. Change the skybox itself and experiment with the light! It can add a lot.
4. I think you should see if you can add a bigger variety of colours and textures in the map. Not only because it's very grey looking, but also because a) People really like colours these days.. Just take a look at the new maps; "Zoo" and "Log". b) because it will be easier to memorize spots and areas of the map if you make them more "recognizeable" and unique.

I hope you'll find my feedback useful. ^^