with levels the best place to start is shape... texture detail unless it calls for something more complex then use a sperate model.
worlds themselves should always be kept relatively clean. this is mainly so the player can move around without horribly inaccurate collision to keep the portals clean.
what you want to try and do is create worlds where your just making the "rooms" for people to be in... the main detail should come from models.
Look at other FPS for inspiration in this respect.
Personally before I set out on creating an FPS level, I know sketch down major locations before trying to create it; then I'll let my imagination help me figure out how to link it all together.
Trying to be creative about it while making the player feel that the area is real and function is hard. When I first started in Quake like geez .. well back in '94 I used to just do it by-eye; but you get overwhelmed quickly.
It's also hard to optimise. While sections might look good, you have to think about how it'll composite in your head; what sort of end performance it'll have.
So it does depend on the engine what can be handled. Doing some test levels first might be a good idea.
