Serbia, Montenegro, and Macedonia are in (congrats Besir!) but Albania, Bosnia, and Kosovo are out (sorry Arta). This is a mess:
This leads to some interesting weirdness. If you’re an ethnic Serb living in Serbia? Come January 1, you’re good, no problem. Ethnic Serb living in Bosnia? You can’t travel to the EU on your Bosnian passport, but you can easily get a Serbian passport that will let you fly like a bird. Ethnic Serb living in Kosovo? Too bad — you can get a Serbian passport, but it will be the special “Red K” passport that will trigger alarms if you try to cross an EU border.
Ethnic Albanian living in Kosovo? Same drill — you’re stuck in Kosovo. Ethnic Albanian living next door in Macedonia, Montenegro, or Serbia itself? Congratulations! You’re free to go.
Obviously there is going to be some sudden border-crossing in the next few months. Albanians in Albania and Kosovo will suddenly discover roots in Macedonia; Serbs in Kosovo will suddenly develop addresses in Serbia proper. Nationalists on all sides will construe it as evidence that their side is right.
The non-Serb, non-Croat (Muslim) Bosniaks get screwed the most: they don't have another "ethnic homeland" to claim heritage in and get travel permission.
The reason for the pickiness is the fear that poor citizens of Balkan states will flood the E.U. with migrant labor. Arguably, Serbia, Montenegro, and Macedonia are in a better position to stem that tide than the others. While visa-free travel permission does not come with work permits, there are real reasons for worrying that that functions as a mere formality. The E.U. would prefer not to have a flood of undocumented workers, for obvious reasons.
But there could be another reason for being choosy. The three countries left out of the new policy -- Albania, Kosovo, Bosnia -- have Muslim majorities (gasp!), so while the E.U. isn't exactly saying that Muslims aren't wanted, they're sorta saying that Muslims aren't wanted. At least, that's how some Europeans are seeing it.