I'll go ahead and throw my 2 cents in here. TK is pretty much on the ball with his net map suggestions.
For lighting, differential lighting is the key indicator of whether I even bother to play through a net map when I'm looking through them solo to see what new ones to use, or whether I just quit out. A level could have awesome flow and placement, but with the same color texture on all the walls at the same light level, it just looks incomplete or unprofessional to me.
Some other helpful tips, in addition to lighting you can use different textures to define edges. Like, if you've got a column in front of a wall, use a different texture on the front of the column from the wall behind it, or if you have perpendicular walls if you use two contrasting textures, the line where they meet up is better defined. Try to avoid making wall and floor textures too similar, or it's hard to see that edge where they meet up. Same goes for steps; if the front and top are the same texture/lighting, it's really hard to make out that they're steps.
This is another example of how maps that try to look realistic (since in real life all the walls of a building are made of the same material!) don't work out in the game world.
Also, vast areas of texture get boring if you don't break them up. If you look at the coriolis loop, for example, most of the long hallways have lights or stripes along the walls or ceiling or things like that, which both look good and also help you relate to how long the hallway is, since you can't use depth perception.
Well, just thought I'd jump in with that.
