It's a bit...well, plain. You could add a bit more architectural detail (in the form of brushes) where you now have plain or tiled textures, just to make the flat surfaces a bit more varied and 3D. At the very least, split some of those huge flat expanses up with trim textures. Just give the whole map a once-over like that and I'll be happy.
The drop-off into space at the edges is funny, too...I suggest a portal sky with some brushwork buildings to break that effect up.
OK well I've been adding some detail in here on the walls and such, and here are a couple of updated shots. I stuck with the detail brush buildings around the outside rather than create a new skybox because they look more 3d this way. anywhere you see a dropoff into space, will most likely be covered by the other brushes when i copy and flip the blue side. also, there is a funny artifact in the upper right corner on the first few shots, so ignore that, as my winamp playlist spilled over my JA window and messed up the SS process
yea I know it doesnt look that great, but Im trying to design it for good Gun gameplay as well