As an MMO level designer I think you have to consider what the area will be used for. There are certainly more different areas in an MMO the there are in a single player shooter.
In WoW you have your cities which are social hubs so you have to remember theres going to be 100s of people in that area. You have to have memorable places etc etc.
in a dungeon you have to consider other things like AOEs, will they aggro other NPCs near the party. layout of structures etc.
A good tip is to go through your favorite game and write down what you see etc. you can even draw out your favorite city in an MMO you play