Why did the Greeks consider Macedon "semi-barbaric" and Epirus as more barbaric than Macedon?

The answers here are either incomplete or improperly informed, so I'll help you out on this one. Quite contrary to what /u/rstone2288 claims, the Macedonians were not considered barbarians because of their subjugation by the Achaemenids. First of all, the Macedonian state was a client-state of Persia, not a satrapy, following Mardonius' successful attack on Macedonia and Thrace in 492, although they had been in alliance since at least Persia's nominal conquest of Thrace in 513. During the Second Persian War Macedon remained a client kingdom, assisting Darius in his passage through northern Greece and Thrace under duress, but Alexander I in good Macedonian form tried to play both sides (the Greeks thought of the Argeads as being treacherous and untrustworthy) and provided a great deal of assistance to the Greeks, including personally informing the Greek army encamped in the Vale of Tempe in Thessaly that the Persians could easy march around the Vale and outflank them, allowing the Greeks to retreat to Thermopylae. When Mardonius finally fell back from Macedonia he left the state of Macedon more or less independent.

The reasons for the Greek view of the Macedonians is pretty simple. They kind of were a bit backward. Now when I say that what I mean should be very clarified. First of all, there are actually two Macedonias, Upper and Lower Macedonia. Lower Macedonia is mostly the coastal region and it's separated from the Upper Macedonian highlands by a ring of mountains, through which there are only a handful of passes. Upper Macedonia was inhabited almost entirely by non-Greek speakers, mostly Pannonians and various groups related to the Epirotes, Molossians, and Illyrians. Throughout the history of the Macedonian state the upland barons owed some sort of allegiance to the lowland rulers, although just to what degree varied pretty significantly. To the Greeks, and even to the Macedonian rulers, these people were very much barbarians

Lower Macedonia is a different matter. The exact composition of Lower Macedonia is pretty knotty and very confused, but it seems mainly to have been inhabited by Thracians and speakers of the Macedonian dialect of Greek, which is extremely backward and archaic. Here's the thing, though. These people, although they spoke a form of Greek, were not really accepted as civilized or Greek by the Greeks proper. Although they shared many traditions with the Greeks and were probably related rather closely with the people who brought the Doric dialect into the Peloponnese their customs and speech were extremely backwards by the Classical Period. The Macedonians preserved many customs that had died out sometime during the Dark Age, or sometimes even earlier, and in many cases their practices were more akin to those of the barbaric Thracians than to Greeks. These were people who preserved the old function of the wanax, the lordlings in the system of vassalage we see in Homer, long after they died out elsewhere. They preserved the tradition that a man (presumably this was for aristocrats) had to wear a red sash around his waist until he had killed a man in battle. After reclining at table was introduced some time in the Archaic Period or later, diners had to sit at table and were not allowed to recline until they had killed a boar on foot with no traps and nothing but a spear (poor Cassander, although an accomplished hunter, was still sitting in his thirties). The Greeks went so far as to say that the Macedonians didn't mix their wine, which was something only barbarians like the Persians and Thracians did.

So in Greek eyes even the Lower Macedonians were not really civilized. What of their rulers? Well, at the Olympic Games in 500, Alexander I was allowed to compete in the foot race after he produced a family tree tracing the Argeads to the Temenids of Argos and eventually back to Heracles, making them Greek. It was a somewhat grudging acknowledgement, mostly on political grounds, and even as late as Demosthenes we find a refusal to really accept that that makes them really Greek. By the Classical Period most Greeks seem to have accepted the story, even if they didn't accept that that made them real true Greeks--Herodotus tells the story of how Perdiccas and his brothers fled from Argos to Illyria and Macedonia with a perfectly straight sense of honesty (although it's debated just how the ruling class was related to the Greeks most scholars think it's more likely that they were related to the Dorians who swept south, rather than any pre-Dorian refugees). So the ruling class, at least, was acknowledged to be sort-of Greek.

Basically, it's complicated, partly because Macedon was a state comprised of a very small number of rulers lording over a vastly more diverse group of countless cultural groups and sub-groups. The ruling class, although nominally Greek, was considered treacherous and untrustworthy, resembling in their morality barbarians like the sneaky Thracians more than Greeks (they really kind of were untrustworthy. One scholar of the Peloponnesian War handily provides a chart to help readers figure out who the hell Macedon was allied with at any given moment in the war--sometimes they changed allegiance several times in a year, depending on who was winning!). Although the Macedonians had accepted a lot of Greek customs, or preserved a lot of customs that the Greeks had abandoned long ago, that didn't really cut it. As Green notes, the veneer of Greekness even on the ruling class was very thin, limited more or less to a few very artificially contrived customs that had been adopted, the awkward use of Attic or koine among polite company, and the army, which had adopted even before Philip some customs used by the Thessalians (/u/rstone2288's characterization of the pre-Philippic army is extremely unfair and very inaccurate. Although infantry levies in most of the baronies of the Macedonian state were primarily light infantry carrying slings and javelins, the Macedonians had adopted early in the 5th Century a corps, several thousand strong, of "Foot Companions," equipped and fighting as hoplites to serve as the backbone of the army)