As it's a work in progress, I won't spend much time on it.
Content
Make sure it's packaged correctly.
Theme
Try to make clear the theme some more.
Texturing
Let the textures fit the geometry (and visa versa).
Geometry
Keep the wtr low, yet detailed.
Lighting
1 Main light at day-light themed maps is enough. Just put a few more at places where it's necessary.
Detailing
Make sure the details aren't in conflict with the theme and fit very well.
Clipping/Noclipping
Try to add a ceiling of clip material also, so players really can't get out of the map at any time.
Flow
Navigate through the map and jump around all the time. Every time you hit something, you can be sure it should be modified so players won't hit it.
Gameplay
Keep in mind, players should run through the map to get pickups. Avoid situations where weapons are being spawned on the place where they can be used at best. This often makes the player won't leave that certain area.
Overall
I hope that helped a little bit.
