Acceptable Religious Targets: Islam is presented as a false schism of Christianity by Dante, a move that audiences found uncontroversial until the twentieth century brought the Comedy to a far more Christo-skeptical audience.
Ass Pull: At one point, Dante uses a cord around his waist, never described before, to lower himself and Virgil into a section of Hell. Scholarly opinion is still divided as to whether this cord was badly established or was a result of Dante wearing a Franciscan habit.
Bizarro Episode: While plot and symbolism are often integrated in the progression of the Comedy, Purgatorio Canto 29 and 32 both stop Dante's travels in their place to have him witness a series of strange events with no bearing on the rest of his journey. The only significance of this events are as symbolic tellings of the history of the Church, but this is the only place in which symbolism occurs with no parallel plot advancement.
Complete Monster: Dis/Satan was once "the highest of all creatures", above every angel and man gifted by the Highest Joy with immortality, invincibility, super intelligence, and perfect happiness. However, Dis came to love his superiority to the lower angels and only that superiority, making the supremacy of his Father unbearable. In acting on this pride, he convinced his fellow angels and the first humans to rebel against the Love known as God. With his rebellion, the Devil introduced all the evil, suffering, and death that would ever be into the world while condemning any who followed him to Inferno, a realm where beings built for eternal Paradise have been corrupted into murderers drowning in the fiery blood of their victims and cannibals biting into the corpses of their children. With his angelic intelligence, Dis knew all of this would happen if he carried out his "arrogant rape", yet brought all of the tortures and crimes described in Inferno into existence without any remorse.
Designated Hero: After spending so long being extolled for her virtue, Beatrice spends her long-awaited introduction in Purgatorio coldly insulting Dante for missing his old mentor and openly admits that she wants to make him cry in sorrow for his mistakes after her death. The dissonance between expectation and reality is so strong that it is almost certainly an Intended Audience Reaction meant to show how unpleasant repentance can be. With more context from Paradiso, most readers come around to Beatrice.
Ensemble Dark Horse: Francesca, an adulterer damned for lust in the Second Circle of Hell, became the subject of a huge amount of Fan-Art and other Fan Works in the 1800s for the unique Genre Shift to romantic-drama presented by her brief appearance.
Everyone Is Jesus in Purgatory: Italian literary critics have dissected the poem word for word over the centuries finding new meanings for each verse. And knowing Dante's love for allegories, they might be partially right.
Fandom-Specific Plot: Fan works about the Comedy generally involve creating a new canto where Dante and Virgil enter a new circle to meet modern figures guilty of sins Dante overlooked. Examples include "Canto 16.5"'s take on indifference, the condemnation of pretentiousness found in "A New Bolgia," and the shocking contents of "The Missing Canto."
Fan Fic Fuel: The concept that there are Circles of Hell for each sin humanity commits against God has lead to a host of additions, modernizations, and homages of the Inferno in order to make a case against a particular sin that bugs the author. These types of fics go back to at least 1902.
Genre Turning Point: In addition to codifying the Italian language, The Divine Comedy introduced a new kind of epic poem, one anchored in the poet's life, emotions and experiences rather than some great epic story with endless battles. By writing it in terza rima and vernacular Italian, Dante allowed poetry to have a popular audience and invented the idea of a national literary tradition since every European writer and artist sought to be like Dante and write the great work of their culture.
Inferno: Dante climbing through the dark woods, before meeting Virgil and walking through the Gates of Hell, adorned with "Abandon all hope, ye who enter here." The most well-known quotes from the Comedy are generally found here.
Paradiso: Quite fittingly, Dante's final approach to God, where he first sees all the saints gathered together before entering into incommunicable awe, is one of the most iconic parts of the entire epic. Special props to Gustave Doré for his famous illustration of the Rose of the Saints which is often used to represent the Comedy as a whole.
The sons of Count Ugolino, for their father's crimes, were trapped in a tower with him and starved together over the course of weeks. Despite their pleadings, their father failed to speak to them for the entire duration of their death's, even when they desperately pleaded for him to eat them to survive. Ugolino himself says that if this story fails to bring tears to your eyes, nothing will.
The story of the suicides, particularly the one who committed suicide after being imprisoned for a false charge, are especially tragic because of their deformed state in Hell.
Dante gets the chance to meet his great-great-grandfather in Heaven and learn of what Florence was like before the politics and greed corrupted it into what it was in his day. The whole chapter is painfully bittersweet in its description of a community lost to hatred, a pain only made stronger when Dante's ancestor confirms that soon Dante will be forced out of Florence and never return home.
They Wasted a Perfectly Good Character: Paradiso Canto 30 has the honor of holding Beatrice's final piece of dialogue, yet it ignores the complexities and wonders of her character by instead having the lady of Heaven's final speech dedicated to telling us corrupt priests are bad. This is hardly a unique exit considering Sinister Ministers had been condemned at length by Virgil, Pope Nicholas III, Brother Catalano, Pope Adrian V, the chariot-dragon, Thomas Aquinas, Saint Bonaventure, Emperor Constantine, and Saint Peter before Beatrice tackled the topic—in Canto 29, right before she tackles the topic again in Canto 30 before never speaking again.
Ulysses is in the Eight Circle for his trickery with the Wooden Horse and for false counsel during the Trojan War; while the Greeks admired his cunning, the Romans despised him for his deceitful nature during the War, since they believed themselves to be descended from the Trojans. Heck, that final suicidal voyage that drove Ulysses and his men to their deaths? Dante's own invention. Though it should be noted that Dante would not have access to Homer's epicpoems when he wrote Inferno only Virgil's The Aeneid, where he is dubbed "Cruel Ulysses".
Brutus and Cassius are second only to Judas and Satan himself in their punishment, each being gnawed in one of Satan's mouths, while Julius Caesar himself is in Limbo. Given that Brutus and Cassius assassinated Caesar out of fear of his becoming a tyrant, while Caesar conducted ruthless and bloody wars of conquest to make himself ruler of Rome, modern eyes are much less likely to see Caesar's assassination as quite so black and white as Dante did.
For those who don't share a medieval Catholic vision of the afterlife, or medieval Catholic ideas of right and wrong, the punishments can come across as Disproportionate Retribution in the extreme.
Paradiso places two nuns who were forced into marriagenote one of the nuns calls herself a virgin, leaving rape out of the picture. in the lowest sphere of Heaven, since the marriages broke their vows of chastity. This is abhorrent to modern readers, who naturally question how unconsenting victims could put in a lesser place for having evil done to them. Dante's guide answers these concerns in two ways: one, the nuns normally sit with every other saint and angel in God's Empyrean and two, the guide claims that the nun Piccarda failed to "absolutely" will to avoid the marriage.
This second answer brings us back to Values Dissonance, since the modern thinking about sexual assault makes it quite clear that the victim is never, ever at fault. Even if Piccarda was never assaulted sexually, the idea that she was in some way responsible for a kidnapping that she didn't consent parallels the victim blaming unjustly faced by victims of rape. Dante's depiction of Piccarda is well-thought out, but that doesn't ease the moral gap between her fate and modern ethics of consent.
Even after seven centuries that saw Christian society change radically, The Divine Comedy continues to be admired throughout the Church for its genius portrayal of a life that begins in sin and misery, strives to do better, and ultimately finds rest in Love. It is difficult to find a better endorsement for an author than to have the Pope call you a "prophet of hope" when your poem sets a few Popes on fire.
Although he reaffirms his respect for the Papacy and the Church's authority, Dante spares no venom when cutting down the corruption in the priesthood and religious orders, and has no problem using the Saints as mouthpieces for the sake of this condemnation. Hearing this from the greatest Catholic poet of all time earned the work special appreciation from the Protestants who sought to reform the Church and those struggling with faith in the wake of the clergy sex abuse scandal.
What an Idiot!: Virgil is told by the Always Chaotic Evil devils damned in the circle for Consummate Liars that the bridge out of their circle has been destroyed and that it is now impossible to move on past this shortcut the devils know. You'd Expect: That Virgil would be ignore the literal demons and take the path God has lined out. Instead: Virgil takes the devils advise and goes their way. The Result: The devils attempt to kill Virgil and his charge, forcing them to jump out of the circle down into the pit of the hypocrites, who make fun of Virgil for not sticking on his original path.