Can I add to your list? 4. Chokepoints and shield doors: Unless desiging for conquest, there should never be a situation where only one way exists through the center of the map between the two teams. This allows one team to control the game by dominating that point and not having to worry about an attack from behind. An alternative to deleting some obstacles to provide more routes is to use teleporters or mancannons to link the two sides (i.e. narrows). Shield doors can ruin a good map if implemented improperly. Never have a shield door with a dead end behind it, meaning that someone camping behind it with a sword/shotty is practically invincible. Also, never have two shield doors within close proximity of each other, allowing someone to camp between them. It is a good idea to leave a small gap below a shield door for grenades to be thrown, or elese put a couple fusion coils behind the shield door to discourage camping.
Actually, improper use of spawn areas can cause serious camping problems (mostly on slayer). Another thing I find important is to make sure every position as at least two ways to access it, and is not to well covered. If a possition is well fortified and is unbalancing your map,you can usually ad a fusion core or two to make it a less desirable location.
I personally don't like snowbound to much because of its sheilds. Someone always seems to be there with shotgun or mauler.