Another important step toward modern literature that Chaucer takes in The Canterbury Tales is his use of irony.
Every writer is a product of his/her time, in one way or another. Chaucer reacts against the literature of his time.
In "The Pardoner's Tale," for instance, he takes a didactic (preachy, or designed only to teach), allegorical form, and reverses it, turns it upside down, so to speak. He takes an often-told tale usually used to didactically preach about the evils of greed, and turns it back on to the tellers themselves.
He does so by using the stock, usual characters--death, drunkards, good-for-nothings--in the story itself, as so many others did before him, but having the story told by a Pardoner that is himself greedy and despicable and sly. This makes the focus of the tale the Pardoner himself, rather than the greedy drunkards in the tale.
This turns medieval, didactic, church-oriented, mostly low-quality literature into brilliant irony.
That's Chaucer. That's why he's still in the text books. That's why his works are a big step toward modern literature.