I would add a fifth one which I've heard somewhere: For every setting element you create, also create some secret element behind it or related to it. This gives the PCs something to constantly discover or investigate. Some of the secrets might be earth-shattering ("the gods are really just powerful mortals?") but many will be minor ("ah, so the mayor has been in league with the bandits all along..."). Don't overdo it: if you can't think of a good secret, don't jam a lame one in there just to meet the quota; the point of this guideline is to create a setting with a lot of subtlety and mystery, not turn the campaign into an ongoing Scooby Doo episode. (As a point of procedure, I think it's easier to write down this secret knowledge in a separate place, so that you can show your "setting guide" to interested players without having them learn all the secrets.)
(BTW, I got here through Johnn Four's Role-Playing Tips Weekly, so hopefully some noob GMs did get exposure to these helpful maxims.)
In an area albinos are considered to be evil mystics and locals ward themselves against them by turning their backs to them to avoid being mesmerized. Suddenly the angeliclly pale loner with white hair and violet hued-eyes is suddenly an outcast, and his companions are treated as if they have been mystically bonded into his service, and could be treated with attempts to intervene or given the same stony treatment. Expect poor quarters, no hospitality and to pay twice as much for everything.