Nah, I didn't find any secrets, I just sorta did a cursory run through the maps to see where you're at.
It's not so much that your maps as a whole need to be darker (the darker lighting in the image I uploaded was just one of many ways you could light that area), but rather that you need more "differential shading" between areas. I sort of see lighting as serving two separate functions in Marathon.
Firstly,
shading can be used in specific areas to add clartiy for the player.
Secondly, giving different areas throughout the map different shading from eachother (ie, making some rooms/halls considerably darker than others) helps to make the player feel like they're progressing through the map, and makes different areas feel more meaningful and distinct.
Anyway, I wouldn't say that you have poor taste in textures, it just takes experience mapping to figure out which textures work better with others, and what sorts of surfaces different textures look nice on. Try texturing the same area in drastically different ways once in awhile. It will also help once you start adding more detail to break up your walls and hallways. I don't really have any recommendations for specific changes, other than maybe trying to add some detailing in the outer ring just for fun. It's a very small and symmetrical map, so it would probably be more worthwhile making an entirely new map based off of it, with a bit less symmetry and some more complexity. I think Biohazard by RyokoTK from Paradise Lost is a pretty good example of a map similar to yours, but with more detail and complexity.
Just keep mapping. And keep in mind that when I talk about detail I'm mostly thinking of simple things like a small row of lights at the top, bottom or middle of a wall; not a large mess of polygons.