Two of the concepts that I rather like about the new D&D 4e world-building assumptions are “points of light” and “the world is old”.
The first assumption is that there aren’t any nation-states. Every border in the world doesn’t naturally and obviously abut the border of another nation-state. There aren’t necessarily frontier guards and passports needed everywhere, because the ‘borders’ of civilized areas just sort of fizzle out into wilderness that doesn’t really belong to anyone, and is therefore dangerous: dangerous to be in, dangerous to control, dangerous to try to hold; and dangerous to enter, visit or travel through.
Instead, we have ‘points of light’, which I take to mean city-states, independent towns, freeholds, and the like. A band of people, Big Men or leaders, and some take-charge middle-managers with some connections to the divine or the arcane, have established a zone of relative safety within that vast and howling wilderness of midnight. The leadership holds a rough circle of territory about 15 miles in radius, because that’s the distance that their pocket army can cross on foot in a day; in mountainous country, maybe they’ve got less. Could be that you have twelve or fifteen of these pockets strung together along a river or a road network, and that’s called a ‘kingdom’. Leave the road or the river’s safety, or leave your village at night, and bad things are likely to occur. You will probably be eaten by a grue.
On the other hand, we have the assumption that “the world is old.” By my count, there’s indications in the Player’s Handbook that there’s been a human empire, gone about 2-4 centuries; a dragonborn empire gone about two thousand years ago, a tiefling empire, gone about a thousand years back, and in between a dwarven empire, a Feywild empire, and maybe a multicultural shindig stuffed in there somewhere. And way back in history, there was a war between gods and Primordials (hmmmm, it’s nice to see that people have been reading thier Greek myths, and Exalted), and a time when the Giants had the dwarves as slaves, and a time when the Fey were all one race, but divided in three (Eladrin, Elves and Drow).
In essence, you have six or seven thousand years of history squeezed into this worldview, and it’s all gone to shit. The assumption in D&D4e’s basic mindset is that you’re living in the times between the civilized, successful, prosperous empires, when collapse has taken civilization down to some of its root layers.