Okay, level design tips.
1. If you are making rooms like bathrooms or closets, keep them small unless they are public rooms.
2. Place entities according to what room you're building; a coffee table wouldn't look right in a cellar, now would it?
3. I'd recommend making warehouses huge; that's what they are, huge. Place girders, double doors, etc. where needed.
4. When you have a big room, align your ceiling lights to the ceiling, and press B a few times. It'll change to grid mode. Place your sources and the lights themselves like that.
5. Use the 'No shaders' option. I personally think my game looks better without full shaders.
6. Finally, look at other people's levels to get an idea of what yours should look like after an hour of level making. I do it; it works.
Failure to Cooperate
Coming to a PC near you.