1) Settings grown organically generally seem to work better than ones designed from the top down.
2) A game world should provide for an incredible diversity of environments and experiences, that way there should be something for everyone. You want to constrain the imagination of DM's as little as possible eg if the DM wants a game in a tropical rain forest, then provide land for such an adventure.
3) You should concentrate on the interesting stuff, not the economics of producing lamb in lower slobbovia or the cost of green beans in upper hallack.
4a) Beautiful maps are a must.

4b) Great sounding names are also a must, nothing ruins the atmosphere of a game more than a poorly chosen name .
5) The world should make geological, geographical and environmental sense. If you're going to depart from normal logic, then you should provide an explanation for it
6) The setting should try at being system neutral ie you should be able to use it with most forms of D&D at least. The more you lock out systems, the more you are reducing your potential audience.
7) It should come with some interesting back stories, which even it they're not played out in game, should be fun for the DM to read.
8) It should provide plenty of hooks for players to become involved (point 9 is part of this ...)
9) The setting should provide political/military dynamics of what might be happening now and into the future. A lot of worlds are presented as being static.
10) Don't base your fantasy cultures too closely on earthly cultures. The closer you get to a real world culture, the more out of place fantasy elements like orcs and goblins feel.