What's the difference between Greek and Bulgarian Feta Cheese
Having worked for some time as a cheesemonger, I found that Bulgarian Feta was generally saltier and more assertive than many Greek fetas.  I also found the texture to be a bit grainier with the Bulgarian feta and a bit more dense.  However, my experiences are limited in that while I sold five different Greek fetas, I only sold one Bulgarian Feta.  I'll also point out that while Feta is historically made with goat milk, it can also be made with sheep milk or cow milk as well, so one has to take the milk source into consideration also (for a quick overview, cow feta will generally be mellow and creamy, while sheep, and particularly goat feta will be more acidic.  But even then, the brine can completely counteract these generalizations!).