What aspects of society does Chaucer satirize in his portrayal of the miller?

In the Generalto
Canterbury Tales, Geoffery Chaucer describes a miller, a workman who is
crude but clever.

The miller is a "stout churl [coarse person]... /
Hardy and big of brawn and big of bone."  His beard is red, like that of "any sow or
fox."  On his nose sits a wart that is crowned with "a tuft of hairs."  His
nostrils are "black and very wide," and his mouth is "like a furnace door for
size."

His behavior is no more attractive than his appearance:

He was a jester and could poetize,
But mostly all of sin
and ribaldries [obscene stories].

He is also a cheater in
his business.  For one thing, he steals "corn," meaning grain that he can grind.  For
another, he charges "full thrice...his fees"---three times more than the fair price. 
Finally, he has "a thumb of gold."  This refers to the practice of many millers to
place their thumbs on the scale when they were weighing the milled grain that were returning to
the customer; in this way they could falsely raise the weight and keep some of the customer's
grain for themselves. 

In summary, Chaucer is mocking the image of the
honest, hardworking, clean-living craftsman.  Chaucer's miller is nothing like the blacksmith
described centuries later by Longfellow in his poem "The Village Blacksmith."