Based on the examples of their faults, Chaucer is pointing the finger at materialism, gluttony, and bribery. Of course, lying is sprinkled in there as well. The Friar is fat, loves hunting with his horses and fine dogs, and does not spend time with his "flock" than in finding his own pleasure in worldly activities.
The Parson, by contrast, is one of only three truly positive characters in this writing. The other two being the Plowman (the Parson's brother--their parents reared them well), and the Knight (probably only because he was the highest ranking member of the pilgrimmage). The Parson said himself, "If gold rusts, then what is iron expected to do?" He is using a metaphor. Gold=the clergy, the ones who are to lead by example and live most closely to the example Jesus left. Iron=the common people who look to the clergy for guidance and help with life's challenges. So, if the clergy are corrupt, how can any of the common people even hope to make it to heaven?