In The Canterbury Tales "Prologue" what suggestions/criticisms does Chaucer share about Medieval society through the positive portraits of characters?

Chaucer is considered a student of human nature.  He was
very observant about what he saw transpiring around him and used his writing to convey
praise or disapproval.

While Chaucer was very unhappy with
the hypocrisy he noticed taking place around him (especially with members of the
Church), he was quick to praise those who were humble, caring, honest and
giving.

In particular, one character he praises is
the Knight. He is a humble man who has served in battle
with valor.  He has been to the crusades, he does not dress to impress, but the clothing
beneath his armor is worn from extensive use.  He is humble and well-mannered, and in
coming home, his first obligation is to to go on a pilgrimage to give
thanks.

Praise that Chaucer would offer by inference
regarding the Knight would include advice that a knight would have to honor his rank by
fighting, and fighting well.  This kind of a man would be expected to handle himself as
a gentleman, not try to impress people with fancy clothes, and humbly present himself at
a holy place to give thanks to God for his safe return.

By
praising the Parson, Chaucer draws
particular attention to his  strengths, and by addressing the righteous things the
Parson does, he draws attention to behaviors that are directly hypocritical to God's
"calling," specifically with other men of God such as the Monk and the
Friar.

The Parson is a man who has taken his vow of poverty
seriously (as the Monk and Friar have not).  Everything he teaches comes straight from
the scriptures for the benefit of his parish.  He is patient in the face of difficulty.
 He does not ask the poor members of the parish to give him money (as do the Monk and
the Friar); instead, the Parson would give to the poor from his own "goods" and "Easter
offerings."

The Parson would work tirelessly for his
"flock," never failing to visit those sick or in need, despite the poor weather.  He was
not interested in entertaining himself or traveling for fun, but stayed close to home to
be of service to his "sheep."

The Parson is very different
than the Monk or the Friar who hunt, travel, take money from the poor, and are intimate
with the women. (The Monk does some of these, the Friar the others.)  The Parson
welcomed all men and women regardless of their lot in life, and lived a life that set a
good Christian example.

By extolling the virtues of the
Parson, by example he points out what is admirable in a man, and what is worthy of
censorship (as with the Monk and the Friar).