The good:

Layout - Spacious, seems like it would be a fun frag.

Archatechture - Nice and clean. very good military feel with subtle use of curves where convenient but not the blatant over use that sometimes crops up.

Items - maybe a tad heavy but I thought it was a good balence.

The not-so good:

Lighting - Monocromatic = bad. Even if you don't want to go into crazy colors which might be out of place you could use lights that are just tinted differently. This happens in real life. For example outdoor lighting is typicaly sodium vapor lamps (yellow-orange), indoor is typicaly mercury vapor (bluish white, otherwise known as flourescent lights). plus there is always room for the ol red warning lights. Your use of the lights you have was damn nice though.

Textures - This is where you really have a problem. That damn concrete wall textures is everywhere. It just makes averything look alike. Technicaly however everything looked great, no misalignments or anything.

Over all I'd say this is really good for your second release. God knows mine wasn't this good.

From here it doesn't look too bad. The design looks a little too simplistic, but I can see the beginnings of some quite interesting designs. Keep going in this direction, with multiple entries/exits, level-over-level design, and a focus on gameplay, but don't limit yourself to arena maps. Try to apply these ideas to bigger, more complex maps and you could be on the road to sweet success.