Warren Okuma said:
Why? It's only a dungeon. A really big dungeon. Best move on to fight epic monsters and such. Loot the nation and move on.
Well, if you want to do it the
boring route, sure.
But at least in my campaigns, the point would be this: You have the power of life and death over tens of thousands of people. You can try to shape their society into something more to your liking. Will you turn your kingdom into the shining jewel of the world - or will it collapse under its own hatred, intrigues, and corruption?
I mean, why
should the PCs "move on"? Are their goals in life really so petty that they are just about slaying bigger monsters and looting bigger treasures? Or would they grasp power over other people if they get the chance - power they could use for good or for ill?
Nation-building offers a vast range of adventure possibilities even if the PCs are the most physically powerful people in the world - for combat power does not equal the wisdom you need to be a leader of people. If the PCs have a strong code of ethics, they will need to restrain their power, or they will be seen as arbitrary and no longer able to be lead by example. And if they decide to rule by just bullying everyone into submission, their subordinates will act this way as well, resulting in a nation defined by the rule of the strongest. And, of course, any effective ruler requires faithful and competent subordinates to manage the smaller stuff - but where will they be able to find people they trust?
Really, treating whole nation simply as "yet another dungeon" is wasting a grand opportunity for role-playing for the highest stakes. When the PCs reach levels that are sufficient for toppling nations, it is high time to let them face the responsibilities that come with power.