Linguistic and Ethnic Diversity
There’s a lot more, of course. One bit I find really interesting are the relations between the center and the Partisan movements in Slovenia and North Macedonia, where the main language isn’t Bosnian/Croatian/Serbian/Montenegrin.
Slovenians got started with the business of organizing resistance quickly, and were among the first to form their own regional antifascist council. Their Partisan movement was at the same time both pro-Yugoslav and quite autonomous, and successfully maintained an antifascist popular front that was led by the Communists, but not dominated by them. The initial liaison sent by the Supreme Staff almost blew it by trying to dictate Communist uniformity and strict central control.
Macedonians eventually had their Partisan resistance, too, but it took a while to get going. First, they needed to be assured that their ethnic identity would be recognized within a Yugoslav movement, as pre-war Yugoslavia treated them as wayward Serbs or crypto-Bulgarians. Second, because most of the area was occupied by Bulgaria, and the Macedonian language indeed is much closer to Bulgarian than to Serbian, arrangments needed to be made with Bulgarian Communist-led resistance on who would organize a movement where.
As for what took place in Albanian-majority areas in Kosovo and Western Macedonia … I have my guesses, but I won’t speculate without reading up on it. This would be really interesting to delve into, as the unresolved status of Kosovo was one of the main points of contention that brought down postwar Yugoslavia – and eventually presented a serious challenge, also still unresolved, to the post-Cold-War global order.