I'm not talking about full detailed lighting. Basic lighting will do, as long as there's light and the map isn't stuck in fullbright mode. The latter will break the server, so we won't test your map on the gameplay of it.
Basically, it comes down to this. Highlight important areas (doorways, paths, objectives), make sure rooms are lit properly so you can instantly see what your enemy's playing as. Even if it's not finished and you're doing your own internal testing, get some light entities in the map. That way you can test for both layout and player visibility (and also if a route is properly and instantly visible).
This is my map currently in development. From this image, you can instantly see where the routes are, as they are brightly lit. This makes sure players know where to go without much hassle, and won't get lost or don't use a route at all because they don't see it.