Getting the lighting/overall visual appearance of a map can be difficult. There are many factors, obviously first being the lights themselves in the map. Post process settings and lightmass settings in the world info are also important, specifically these can help fine tweaking contrasting/shadows, which can help a map feel less bland. Something that I find fun, is to open other maps and see what it looks like on your map if you copy over post processing/lightmass/lighting/fog settings. You can then tweak it further to fit your map specifically.
This can be easily achieved by literally just opening another map, finding the main light(s) and copying them, then just pasting into your map. For the post processing/lightmass settings, you can right click the top of the section and copy the settings, paste it into a notepad, open up your map then right click and paste the same section on your map's world info.
Besides the map itself, you can also change some material parameters to make specific assets look better on your map.
Disclaimer: I'm not a mapper nor an artist. That being said, I have done a fair amount in the map editor, so this is really just what I picked up over that time.