I used to write a lot more about world-building on this blog. Now it's all pretty pictures. Really, I do still think a lot about campaign design...
Memo to self: if you're planning a fantasy campaign, consider checking your world design against these rather broadly-asserted maxims:
1. Balances of power are only interesting in the past.
Don't establish a balance of power between powerful entities in the game world unless you are going to remove it immediately as a way of drawing the players into the story. Here are some premises that you should probably change from "might happen" to "just happened" as a way to have the PCs hit the ground running:
Two hostile superpowers which are only a spark away from igniting a planet-wrecking war.
A web of alliances between nations or groups of nations that could unravel over a minor dispute between two parties.
A supernatural or divine balance between two sets of deities that is foretold to collapse and bring about an apocalypse.
I'd like to play in a campaign that starts with the PCs' home city being burned to the ground just as they're about to hear the location of the nearest dungeon from the friendly barkeep. Or maybe it starts in a refugee camp, or with the PCs wandering through an apocalyptic landscape.
2. Kill Gandalf on page 1.
Leave some ridiculously large boots for the PCs to fill.
Don't have NPCs who are more capable of solving the world's problems than the PCs are. I recommend axing your Elminster/Gandalf/Dumbledore as soon as you've established how wise and beloved he is. If you don't, sooner or later your players will be asking you "Why can't Dumbleminster fix it if he's so freakin' powerful?"
3. Don't waste time with micro-level world-building.
By all means, do some general campaign planning on a global scale.Sketch out a node-based map of the world with maybe a paragraph about each area. But don't waste your time fleshing out the world much beyond where the campaign starts. By the time the PCs get there, you will have better ideas, trust me, most of which you've stolen from your players' wildest guesses. I run into this every time I've played an extended campaign -- your ideas will suck unless you are in a panic that your PCs will be exploring the place sometime in the next gaming session.