I like to include Romans, Vikings, Gypsies, Mongols, and Comanchee/Sioux Indians. Each has a very distinct feel compared to the others. Yes, combining them is anachronistic as hell, but they are the historical cultures I'm most interested in. I also include the Broo from Glorantha, in all their evil, disgusting glory.
In general, I favor settings where the various countries and areas are easily comparable to real world cultures. It provides a great base to build off of. It's one of the big difficulties I have with settings like Talislanta or Tekumel, which are so original or foreign that I really can't grasp what to do with them. I suspect I'm not the only person like this, due to the popularity of generic Europe settings.