The Guide to a Successful Playerworld

[Prozac]

plan the work
then work the plan
if you hit some bumps in the road
revise the plan

revolve everything around the big three:
quests, community, and competition,
and how they interact with each other.

and check out lots of other playerworlds to make sure you don't find out that you are unintentionally copying someone else who has basically the same idea you have.

get to know your staff well. monitor how they get along.
no matter how good your plans, levels, npcs and other content is, staff arguments and misdeeds alone have brought down more than one server.