Christ, I get long-winded sometimes. This one will be short.
Anyway, similar to how one should avoid "real" maps, one should also avoid "real" lighting. Maps in Marathon don't always look right when you have carefully measured lighting, and in fact, lighting usually distracts the player's eyes. The primary goal when one uses lighting (or darkness) in a Marathon map is to call attention to something, or to divert attention from something else. Rather than actually having each light fixture cast out a light, you could just have the light fixture and keep it at that.
There's also differential lighting, which is better explained with pictures than just words, and since I don't have any pictures, I'll just link to Hastur's article on differential lighting, which is very well-written and gets the same point across: http://mmmg.bungie.org/hastur/aesthetics1.shtml
So how should you use lighting in general? As I said before, you use it to call or divert attention. In a brightly lit area, something that's dark encourages investigation (hey, what's that dark corner over there?). In a dark area, lighting helps guide the way. If you put lights on the walls of a dark hallway, they tell the player where the walls are, even though the room is still dark (example: "Ne Cede Malis"). If you actually had the lights cast light on the floors and such, you'd not only confuse the player, but you'd also lose the effect of having a dark room in the first place.
My advice on this issue is to try and avoid the extremes of the light selection in Forge - lights 0-3, and 18-20. Use them sparingly, so that when you DO use them, the player thinks "wow, this room is really bright" or "man, it's dark in here." If every single "bright" room is Light 0, then you lose the effect of having the room being brilliantly lit in the first place. The same applies to the other end of the spectrum - don't make every cave be Light 20, because it loses the effect of being a really dark cave, and starts to be annoying.
In netplay, the effect is even worse for darkness. Even when I want a room to be REALLY dark, I don't use Light 20, I stick with 16-18, and then I put the occasional light here and there so you can still get a feel for the room, without actually lighting it - a good principle overall (example: "Morpfhine"). Feeling good is good!
