You've made a couple early-beginner mistakes that, if rectified, could turn this into a quality map.
1) Large, boxy architecture. Make smaller rooms in general han those in, say, the first shot, with the exception of large hangars/cargo bays. And when you do make large rooms, it's generally a GREAT idea to put lots of smaller things in the room to break it up, add more character, make it look more realistic, and provide a more interesting environment for the player. Also, things like that h-u-u-u-ge block wall with the arch need to be reworked or nixed. That thing looks ugly the way it is, it needs smaller detail work and an actual beginning and ending rather than just the brush limits.
2) Repeating textures. For any large area, of which you have many, find textures that are LARGE so they don't repeat as often, or repeat in a more interesting fashion than, say, the ceiling in the first shot. Also, try placing small-brush detail work like girders or struts to break up areas of repeating textures.
3) Ambient lighting. Kill it. Kill it dead. Use no such thing in your map, unless it comes in quantities less than 20. Use entity lights indoors and a light-emitting sky outdoors for this map.
4) Crappy block terrains. Take the time to learn to use EasyGen. It'll be worth it, I promise you.
5) Bad map/skybox seam. Put SOMETHING in there to block the transition from playable map to skybox. An easy thing to do is throw some lines of trees, big cliffs, and walls around the edge of your map. A more advanced technique is to use a foghull.