Well, allow me to inject a bit of realpolitik into the situation:
There are no such thing as international "principles" or "rights" or anything. There is simply what nations can get away with doing.
Kosovo is independent due to the fact that the price for Kosovo to remain part of Serbia is a price that Serbia is unwilling to pay (due to the presence of NATO peacekeepers). Principle doesn't enter into the equation.
The South failed to become an independent nation, not because some court decided not to, but because they failed militarily and diplomatically. If France and Britain had, at the time, decided to support Southern independence, that independence would have likely become real, regardless of any principles claimed by the Federal government.
It is grossly simplistic and naive to think that international relations rely on any particular set of principles. If any "principles" are adhered to, its because there is a practical reason to do so. (Example, the US made sure they had some kind of UN resolution before going into Iraq... I don't think they had a clear cut one, but enough of one that they could claim they weren't acting unilaterally. Its not much, but it muddles the point enough).
Public opinion in the major countries is pretty much the only thing that could have prevented Kosovo from declaring independence (that is, if they knew that their move wouldn't have broad Western support), but after the rather nasty post-Yugoslav wars, sympathy for Serbia is at a rather low ebb.
Plus, as a historian, I have to point out that the Serbians started WW1. :P (yes I know that its more complicated than that, I have to have a bit of levity)