What cultural factors have affected the boundaries of what used to be known as Yugoslavia?
The most important cultural factors that have affected these boundaries have been religion and ethnicity. Yugoslavia broke up into smaller nations that are based largely on these factors.
The best known examples of this are the Serbian actions taken against Croats and Bosnian Muslims. This happened during the early 1990s. A great deal of "ethnic cleansing" took place at this time with Croats (who see themselves as different from Serbs) and Serbs being separated into different countries. Similarly, Bosnian Muslims ended up fighting against both Serbs and Croats because of religious differences. Today, Serbia, Croatia, and Bosnia are separate and relatively homogeneous countries. In this way, cultural factors led to the creation of boundaries breaking up what used to be Yugoslavia.