Being of Serbian descent, let me provide a little historical perspective. In 1389 the combined forces of several Serbian noblemen fought a battle on the Kosovo plain against the army of the Ottoman empire. The battle was truly bloody and both armies suffered massive casualties but the Muslim Ottoman army prevailed.
Hence the Orthodox Serbs and the Muslims of Kosovo have battled off and on since then. For many Serbians the battle is not forgotten. Hence the Patriarch of the Serbian church threatening violence. Something that I find incredible coming from the head of a church.
I agree with Selco that the Balkans are a proxy for the big countries to jab at each other. The sad thing is that everyone was able to live together peacefully until the politicians got involved.