what aspects of medieval society does Chaucer satirize in his protrayals of the merchant? of the Franklin? of the Doctor? of the Miller?

Chaucer really goes after the religious figures of the time. He satirizes them as hypocrites after the money of hard working, if naive, people. The people that you mention above really serve for dirty stories which the pilgrims enjoyed so much. Most of these characters are at least likeable, and less respected by the public, as opposed to the to the real "villains" (the Church) who are more respected by the public.