Are there any games you had in mind that you would like to capture the overall style of? This question is a bit general to really give a specific answer on...
But I'll give a try anyway.
Most people are of the feeling that dark is better for some reason. While I agree that dark is good for capturing certain scenes, how often do you actually see it in real life? I make copious use of lighting, unless it doesn't make sense to not light the scene. Not only is it more realistic, but it also helps to show off your texture work (unless you are trying to hide something). Little bit of light will really liven up the scenes (but don't light crypts, sewers, or other such dreary areas). One of the oddest things that I had ever seen was a lab level that was dark and dreary. It just didn't make sense. Every lab that I had ever been in was blindingly bright.
Not only that, but I feel that a lot of the feel behind FPS games is in the models. They all look like robots. Then again, when you are trying to show a human with less than 1000 polygons, it will tend to look like a robot. Feel free to be generous with your character models. A few extra polygons make a huge difference in the overall form, and that can help to bring out personal connection.