Yes it is, but cultural and religious differences do play a role, don't they?
The differences between the nationalities really aren't that pronounced, but they are enough that the Slovenes, Bosnians, Croats, Macedonians and Kosovar Albanians all wanted to gain independence from Belgrade. And the Montenegrins are trying now.
Just watch our Balkans posters argue over what language is really spoken in Croatia/Bosnia/Serbia; or are the Orthodox Churches in Montenegro & Macedonia separate entities or part of the Serb Orthodox Church; or whether its FYROM or Macedonia or whether Macedonians are actually Greeks or Bulgarians or Serbs. The Albanian national hero is named Skanderbeg--there are posters on here who will argue that he is really a Serb.
So, as an outsider who's spent a lot of time in the Balkans, I'll say that they really don't have many differences, but they sure know how to make the most out of the ones they have!