No, I'm not talking about what happens when you leave yogurt in the fridge too long. I was getting all ready to post about politics, but it sort of dawned on me that culture probably comes first. According to wikipedia, culture "refers to patterns of human activity and the symbolic structures that give such activity significance". I like that definition, it works for me.
Cultures are often regional, but this isn't always true, looking at diasporas (people living outside their cultural homelands, often used to refer specifically to jewish people living outside Israel).
In fantasy, one of the major issues is actually with mono-culture, especially when dealing with non-human races. I'm going to try to avoid this, but my traditional direction may make a mono-culture or two unavoidable.
I also plan to introduce counter-cultures in large centres of culture, of which there will probably be about five human centres, and one centre in each of the other races with significant counter-cultures.