Social class and its contradictions
The pilgrimage mixes every rank of medieval society, and the tales keep colliding with each other across class lines. The Miller undercuts the Knight. The Wife of Bath outargues the Clerk. Chaucer shows that social rank and moral worth do not line up.
Corruption in the Church
The Pardoner, the Friar, the Monk, and the Summoner are all Church figures who abuse their positions. Chaucer does not attack faith itself, but he is relentless about the gap between religious office and religious behavior.
Gender and the question of sovereignty
Who controls whom in a marriage? The Wife of Bath asks this directly. The Franklin's Tale and the Clerk's Tale approach it from different angles. The question runs through the work without a settled answer.
Appearance versus reality
Almost every pilgrim performs a version of themselves that does not match what they actually are. Chaucer's irony depends on the reader noticing the gap. The work trains you to read skeptically.
The instability of moral authority
Corrupt figures tell true moral lessons. Virtuous figures behave badly. The Pardoner's Tale is the clearest example, but the pattern repeats. Chaucer refuses to let moral authority rest safely with any single type of person.