As far as cultures go, I'm kinda spoiled... I DM a medieval Earth setting. That makes things very easy... or does it?
The elves live in northern Europe; they were the ones that sacked Rome, and many of them still remember it. That's one quick culture point.
The orcs live in eastern Europe and northern Asia. They wear furs and have cossack-style dances, which is cool. (The PCs loved it.)
The gnolls come from beyond the Congo, and have converted to Islam. So you've got these hideous massive man-dog beasts that could tear you limb from limb, and instead they police the marketplace or work on mathematics. (Actually, I haven't got any gnollish mathematicians in there yet... I'll have to do that, because at the time Islam was more advanced than anyone else.)
The Mer (any aquatic race that doesn't eat people is a Mer race) show up from time to time when you're sailing, exchange conversation, maybe trade a little. Some of them come on land to trade, so you can see a blue man in a booth on the docks selling valuable shells, for example. (It's like trading glass beads to the natives, heheh.) They're the only non-historical realm in the piece, but they stay strictly neutral and are naturally separate from terrestrial realms.
Dragons... if they're known in a region, they're a power base and leadership rolled into one. It gives a culture stability, I'll say that. "Hi, I'm Razinus Peralin. I once ate a Roman legion. Do you still want to cause trouble in my town?"