...mappers are often people who build buildings in real life. They know how a building is constructed, the shape, the measurement etc
That'd be me some years from now!
Some small mapping tips:
-A good map must have a reason for existing. Try to do something we havent seen before. (sounds hard)
-In deathmatch maps, avoid repetitive and labyrinthic hallways. The players must always know where they are. They need recognizable references in the map to orient themselves.
-A map must consist of different areas of different sizes. Weapon, item and respawn placements have the final word when it comes to game flow.
-Performance is the first objective. No matter how good looking a map can be, it wont be fun at 10 frames per second.
-Balance the map detail. In a small room, you can always place more pipes and other detail that in a big room.
-When trying a deathmatch map, think that other players consist of more polygons, so the r_speeds/+showbudget are not exactly what you see when you are playing it alone.
-Work on the 3 fundamental scales of mapping with the same effort:
1) Theme
2) Architecture
3)Detail
If one of these is wrong, the map wont work or at least it wont be remembered for long.
I'll think of other tips later, I need to go to the Post Office to send some Uni stuff :S