What are some thematic concerns in the General Prologue?

Chaucer's Canterbury Tales was the work that put English on the literary map as a language capable of producing great literature. Before the 'Tales, most great works had been written in Greek, Latin, French, and Italian.
The General Prologue is the first "chapter" in the Tales. In the Prologue Chaucer tells the reader about the pilgrims journey and introduces each of the characters. Thematically, Chaucer gets the reader ready for the characters' tales that follow by describing the various characters' personalities, and in some cases, their moral weaknesses.
Some characters, like the Pardoner, Friar, and Monk, advance the theme of corruption in the church. Characters like the Doctor advance the theme of greed. But there are good characters too, like the Parson and the Plowman, who Chaucer uses to present of theme of simple human goodness based on humility and faith. 
Much of Chaucer's message is delivered with a verbally ironic tone, as he seems to be describing characters in a positive way even when they turn out to be morally corrupt. We are left to wonder if the narrator is just not that bright, or if he just doesn't want to be directly critical of his fellow travelers.