[Awar]

I'm no good with lingustic terminology. There are several Serbian and Croatian dialects which are more archaic, and, like Macedonian and Bulgarian, simply closer to old Slavonic.

For example, the 'change' of 'L' into 'O'. In some areas of Serbia, people still say 'pepel' instead of 'pepeo' etc. The 'regular' Serbian and Croatian have went through several changes and reformations which lead the official language in a certain way, but the regional versions remained where they were, more archaic...