The relations between Bulgaria and Macedonia were a bit more bumpy, I don't know all of the details, but Bulgarians* consider(ed) Macedonians just as a part of their own and their language as a dialect. Serbian and Macedonian languages are far to different for that. Yes, we can understand each other with some trouble, but that is no where near like Croatian, Bosnian, Montenegrin and Serbian which at times sound identical.
The only issue is the conflict between the Serbian and Macedonian Orthodox churches, but that really does not have effect on anything else than themselves.
The situation with Albanians in Macedonia is disconcerting, at least from a Serb's perspective. It is just too easy to notice a pattern of behaviour, incidents and evenrs, and almost threatening stance, even in some comments here regarding the Albanian majority in Macedonia. Serbia has been through that and handled it poorly, and I'm afraid that Norhtern Macedonia is on the same slippery slope.