The same old thing. For example little kids from CZ which have never heard of Slovak might have trouble understanding some different words, but when they're growing up and they see a Slovak actor/actress on the TV or hear a Slovak song, they get used to it. I still don't know all the Slovak words like names for each flower (honestly I don't care about flowers in any language), but I do understand 95-98% of the language and can speak it to some degree. So in the end I could call Slovak a dialect of Czech or Czech a germanised version of the more Slavic Slovak. Nowadays even the Slovak language takes a lot of words from Czech, because it's so close. And I don't speak Serbo-Croatian yet, but from what I've seen the YUGO pack (Serbian, Croatian, Bosnian and Montenegrin) differ even less.
So they could be considered dialects. Other languages call their forms dialects and are much more far apart. So the difference between dialect and a language is the politics. AMEN
P.S. No matter how close Bulgarian and Macedonian are. If they want to call their dialect a language, it's their "thang". If Americans wanted to call their language American, even though it still would be mutually intelligible with the Continental English, they'd have every right to do so.
If L1 and L2 come from the same root and have evolved step by step, it's ok I guess. If one country drew f.e. L3 words just to be different, then it's not that cool. AFAIC Macedonian has these extra letters Ѓ Ѕ Ј Ќ Љ Њ Џ while Slovak has these little bastards ä, ľ, ĺ, ŕ, ô - this proves nothing, but anomaly in ortography. Call it a dialect, call it a language, but no need to use linguistics as a weapon for the means of racism.