One of the most important things in maps (and one of the things, most newcomer mappers don't consider) is the "Size & Scale" of your map. So what is meant?
If you start with one of your first maps, you tend to think in small dimensions, little rooms and tight corridors, because you don't have a lot of space left later where some portions of detail are required. This saves time and work but heavily decreases the value of your map. It's hard to fight and move in such small places and beyond doesn't look that good, if these areas also get cramped (shot 1). So if you create new rooms ore hallways, make them large, large enough that even 4 players could battle demons in there without getting in their own way and later fill them with some necassary detail and monsters like I demonstrated that in the second shot. It doesn't just look better, it even plays better because you have some space to move and dodge the demon's fireballs.
And for the second reason, why "Scale & Size" is so damn important: I can just say you already read this some paragraphs before: "Keen On Sensations!". Just think about large sector arrangements in "Phobos Anomaly Reborn" (Chris Lutz), the huge floating castle in "Torment & Torture 3" (shot 3) or even some maps from the game Unreal (Epic Megagames). These places are so damn large in scale, you will never forget them and surely visit these "sights" once more which makes up for a good replay value! Sure, they need a lot of work and sometimes even tricks but believe me if I say: It's worth the work!

Summary: "Size Does Matter,Cramped Rooms Are Bad Rooms"