Not really BM related, but more map related. Being a CS player (not pro, but not clueless) I can give some tips on how the current maps are formulated. Not that you need them, but to keep you from making some common mistakes I see in many many fan maps.
1. Make everything notable. Callouts are important in mid to upper level play, and it should be easy to relay an enemy’s position. If every room and hallway looks identical, it will be difficult to describe the enemy’s location. A good example is Agency, despite taking place in an office building, every location is notable in some way (from metal detectors to renovation, even the hallways are notable).
2. Keep it simple but distinct. It should be easy for one to get his bearings, a few ‘rooms’ connecting to each other is better than a maze of identical stairways and hallways. A player should be able to look at any part of the map and say “I know exactly where I am”, and even better “I know exactly how to get from here to there”. Finally, there shouldn’t be any areas where a player can be shot from half a dozen places on a map. Areas like that are frustrating and unfun, they play sloppy and give the victim little-to-no chance to do anything.
3. Connectivity and access. There should be multiple paths to most areas to allow for relatively free movement. There should be no way for one person to cover every entrance and exit from one spot. Additionally, both teams should have clear and “safe” paths to each site without needing to pass through the enemy’s side (although they may opt to if it’s faster). A “mid” is a common concept in balanced maps, as it gives both teams a shot at each other from their own sides of the map, while also opening up extra options as T if they take mid successfully.
4a. Bomb Sites. If it’s a bomb map, you’ll want both bomb sites to be reachable by CT’s before T’s may get there. You’ll also want to give the T’s multiple angles of attack, but generally CT’s should be able to hear the T’s coming unless the T’s are sneaking. Bomb sites should also have many obsticles, shot blockers and cover. That side, everything in a bomb site should be distinct, as there will certainly be conflict here.
4b. Keep Hostages separated. I have seen a few very pretty maps ruined by having hostages side-by-side, which destroys all tactical depth and removes any chance of flanking or rotation for both teams. Additionally, you want Terrorist to spawn with or very close to the Hostages. CT should have a relatively safe exit after capturing a Hostage (for example, T should not have access to a place that overlooks the entire route back to CT spawn).
5. Potential design inspiration: The following maps are well balanced: Dust II, Inferno, Mirage, Cache, Ali, Agency. Good maps with slight bias include Nuke, Office, Seaside.