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

In the General Prologue to 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."