GRRM seems to have made a simple but subtle model of cultural conflict/prejudice between Dorne and Westeros (esp. Reach and Stormlands): two different cultures w/ feudal systems (or at least cultures that see themselves as so) that have been in brutal conflict for so long that the populations associate the conflict with the opposing culture. Does Westeros seem to have an odd degree of cultural homogeneity? For centuries, the conflicts seem to be between regions, not between the cultures/faiths.

Well, let’s not forget that throughout the pre-Targaryen period the Northmen are seen as heathen barbarians who send suicide armies to plunder or die trying every time there’s a bad winter, and the Ironborn are monotheist fanatics and slavers in the bargain. So really we’re talking about a core of Andal culture that incorporates the Westerlands, the Reach, the Riverlands, the Vale, and the Stormlands, which is constantly dealing with foreign cultures to its west, south, and north. 

As for regional conflict only, well that’s hardly unusual. The Saxon Heptarchy, for example, for the most part spoke the same language, had the same culture and heritage, and usually shared the same religion (although they shifted from pagan to Catholic). Didn’t stop them from warring against themselves for almost three hundred years before the Danes showed up.