Feminism and Anti-feminism
Chaucer was extremely interested in the role of women in society, and how they reacted to it. In the Wife of Bath's Tale, for example, Chaucer foregrounds the issue of female "maistrie," and in the series of Tales often called "the Marriage group" by critics, Chaucer actively explores the potential dynamics of a male-female marriage. In the Middle Ages, feminism had obviously not been invented; but one sees very clearly in the mouth of the Wife of Bath that ideas of female equality were by no means unusual.
Christianity
The Tales as a whole take place on a religious pilgrimage to Canterbury, and Chaucer's "Retraction" makes a famous apology for the way the Tales have a tendency towards sin. Are they blasphemous? Furthermore, is Chaucer's retraction of them genuine? Critics have argued both cases. But what is certain is that the Tales contain a huge amount of religious material, both in the expressly religious tales (the Prioress, the Parson, the Clerk) and in the supposedly non-religious ones (the Summoner, the Miller, the Friar).
Language and Communication
"What nedeth wordes mo?" ("What more needs to be said?") is a question that is constantly voiced, from the Knight's Tale all the way through the silencing theme of the final tale, the Manciple's. The nature of language, the value of words, whether words can ever have a true "meaning", or whether you can ever really "own" words are all themes which Chaucer scrutinizes at various points during the Tales.
Story-telling
The key structural complication of the Tales is the way that Chaucer situates himself within the fictional pilgrimage, claiming that he is simply recording what other people have said. Thus we are never sure whether any statement is the opinion of the teller (say, the Wife of Bath), of the fictional Chaucer ("Geffrey", as he is referred to in criticism) or of Chaucer himself. It is extremely difficult, due to the dramatic, "ventriloquised" nature of the tale-telling project, to actually ascertain who we are listening to at any one stage.
Fables, Fiction, and Fabliaux
Chaucer is always interested in fables, "moral stories", or genres which have a set pattern - and, to generalise a little, often juxtaposes these fictional, literary traditions with a mode of Middle English realism to see how they co-exist. Thus Chaunticleer in the Nun's Priest's Tale reasons far beyond the means of even the most well-read chicken. Meanwhile, the crow, in the Manciple's Tale, is shunted out of what seems a cartoon-like, fabliau beginning to a tale, to later be physically abused in a shockingly realistic way by the end of it. How does a fictional world relate to the real world? How does a literary tradition match up to the world it represents? Can we ever take a moral from a story?
Vengeance and Debt
There are several instances both within tales and across the structure of the work itself where one character resolves to "quit" another. The Miller, for example, quits the Knight's Tale, only to have his tale quit by the Reeve - and later, the Summoner furiously quits the Friar's Tale with his own venemous anti-Friar narrative. Quitting often provides smaller internal structures within the larger structure of the Tales as a whole, and invites the comparison of one thing to another.
Sex and Adultery
Many of Chaucer's Tales are interested in the way a marriage might work or fail to work. Look at any of the tales which dramatize adultery or cuckolding (the Miller's, the Reeve's, the Merchant's, the Wife of Bath's, etc.), focusing particularly on the way that sexual activity is depicted. Chaucer's presentation of sex varies wildly, sometimes present only through pointed euphemism (like the Wife's bele chose) and sometimes, like in the Reeve's or the Merchant's tales, vividly described.
Justice and Judgement
The Franklin's Tale ends with an explicit question to its audience, asking them to consider each of its characters and then decide which they think is the most generous. It is not the only tale to pose questions and invite comparisons of its characters: the Knight's Tale, for example, asks at the end of its first part whether Arcite or Palamon is better off, and the Merchant's Tale opens with a lively debate between Placebo and Justinus about whether January should marry. Chaucer often puts two things together (this could also be interestingly related to the idea of quitting) and invites the evaluation, the judgement, of one versus the other. Note too the moments in Tales when "justice", be it legal (in, say, the Wife of Bath's Tale) or comic (in, say, the Miller's Tale) is ultimately done: it's clear that justice, in Chaucer's world at least, is not always just.
Parody and Honesty
Also known as "Ernest" and "game," as Chaucer himself calls the duality in the Tales. Many of the comic tales have an undoubtedly serious side or incur serious consequences (the broken arm that John the carpenter suffers during his fall from grace, for example, in the Miller's Tale) and serious tales can often similarly have comic, or ironic moments. The whole tale-telling project remember, is, in the General Prologue, supposed to be "game", but instructive game - namely fun with a moral purpose. Whether the tales fulfill this definition is ultimately up to the reader.
