Just a quick point, but Albanians aren't Slavs. South Slavic ethinicities do however include Serbs, Croats, Bulgarians, and Bosnians. There are also Montenegrians and Macedonians, but depending on what constitutes and ethnicity they are, respectively, Serbs and Bulgarians.

Croatia and Serbia were sovereign medieval kingdoms, Bulgarian Empire was even for some time the main threat to the Byzantine Empire. However Croatia was in 11th century united with Hungary (kings of Hungary ruled both kingdoms) and Serbia and Bulgaria were conquered by Ottomans.

Croats and Serbs are very similar ethnic groups with nearly the same language, but very different history (conflicts with Venice, part of Hungary and Austria etc. X conflicts with Bulgaria and Byzantium, part of Ottoman Empire) and religion (Croats use Latin script, orthodox Serbs Cyrillic).

Montenegrins are basically Serbs, but Montenegro was specific region, thanks to inaccessible terrain they were able to resist Ottomans and eventually developed their own identity.

Macedonians were basically Bulgarians. Serbia, Bulgaria and Greece all tried to seize Macedonia for themselves, but Serbia was the most successful in Balkan Wars (the 1912 - 1913 ones) and Macedonia later became part of Yugoslavia.

Albanians, as was told, are not Slavs and they were heavily Islamized during Ottoman rule. (I was told they didn't care about religion as long as they were allowed to carry weapons - which Christians weren't - but I'm not sure whether it's truth). After loss of Ottomans in First Balkan war Albania became independent - neither Albanians nor Powers wanted to give Albanian area to the winning Serbia. (Albania was never part of Yugoslavia).

Bosniaks are Islamized Slavs (I forgot mention there were strong national orthodox Churches in Serbia and Bulgaria, unlike in Albania or Bosnia). Croatians consider them to be just Islamized Croats, Serbians Islamized Serbs. Bosnia is inhabited by all three nations which caused many problems.

Slovenians were Slavs living in Austria (in Austria-Hungary Croatia administratively belonged to the Hungarian part, but Slovenia to the Austrian one)

Kosovo is important historical part of Serbia, but occupied mostly by Albanians, which also caused many problems.

Whole situation was in addition complicated by wars and migrations, i.e. many Serbs fled from Ottomans and settled in (Austrian) Croatia.

One key factor is recognizing the differing religious, political and linguistic elements in the region. There was, in the Southern areas, a strong Islamic/Ottoman influence, the North (until World War One) had a Hapsburgs influence with different strands of Catholicism and Protestantism, and there was also Russian/Pan-Slavic influence which entailed Russian Orthodoxy. In addition there was linguistic differences inside of these socio-political-religious groups. These differing elements did not simply reside upon the existing ethnic groups but influenced their internal development.

Another crucial point is that National Identity is not something that ever 'forms' in a crystal or empirical sense, rather it is implied, enforced or adopted. Both willingly and unwillingly. The Balkans has always been (rightly and wrongly) viewed as a crossing point in Europe for the East and West; the distinctions in religion, political control and variations in language are all elements that contribute to forming what is classically (Western terms) considered a national identity, therefore the atypical degree of variation in the region has led to it being perceived as more diverse than the rest of Europe.

These perceived differences are largely labels and words, however there is also some foundation in them, the real difficulty is deciding to what extent this is the case. Is the difference between Croats and Serbs a distinction between Orthodoxy and Catholicism, or is it one between a traditionally military and industrial society against a minority intellectual group? The answers are both fundamental but also convoluted.