Greece and Yugoslavia remain neutral throughout the war. Overall, this is better for the Nazis, Yugoslavs and Greeks. Yugoslavia would have spent the war selling food and raw materials to the Germans, building up their infrastructure instead of having it be demolished.
The Germans were forced to maintain 350,000 troops in Yugoslavia, which would have proved valuable, although not decisive, on the Eastern front. They probably would have been rolled over along with the rest of Army Group Center in the summer of '44.
Bottom line, Germany is a little more capable, but still loses. Yugoslavia isn't communist but remains a monarchy to today. Its breakup would still be a good possibility, since the Serbs made some major, foundational errors when the country was founded (by viewing it, not as a union of south slavs, as the other nations (Croats, Slovenians, Bosniaks, etc.) considered it when thy joined, but rather as Greater Serbia, and treated it that way. However, the bad feelings that arose from WW2 would never have happened, and in 1939 Croatia had finally achieved a measure of autonomy. Perhaps this could have been expanded into a true federation. Serbs and Croats have not traditionally been enemies and there is nothing inevitable about their conflict.