Acceptable Religious Targets: Islam is presented as a false schism of Christianity by Dante, which was a common misperception of Islam that existed throughout the Christian world in the Middle Ages. The 20th Century would bring a better understanding of the religion based on its primary sources, as well as a far less orientalist perspective of non-Christian religions.
Alternative Character Interpretation: Given that several of the writer's favorite historical figures appear in the epic, some people have jokingly suggested that this is an early example of the Self-Insert Fic. While the protagonist is an Author Avatar, the entire work is a huge allegory and it's written so that anyone can see him/herself in the traveler.
Awesome Ego: When Dante meets the great poets (a group composed of Homer, Horace, Ovid, Lucan, and Virgil) in Inferno Canto 4, he is welcomed into their group as its sixth member. This could come across as extremely egotistical, as Dante is essentially equating himself with the greatest poets in human history. Except his work is just as well-known and widely-read today as any of their works, if not even more so. Thus, he comes across less egotistical and more prescient.
Big-Lipped Alligator Moment: 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 significances 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/Lucifer 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 of eternal torment where souls are forced to endure tortures such as being transformed into twisted, broken trees; feasted upon by harpies; and being submerged in a river of boiling tar. 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."
Fanfic 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.
Harsher in Hindsight: The first seven "levels" of heaven are the planets of the solar system out to Saturn, plus the moon and sun. Venus (the third level) is mainly about love. Considering that in modern times we've sent probes to Venus and know that it's anything but heavenly or lovely, this is kind of disturbing.
The punishment of corrupt politicians — being trapped in boiling pitch — is especially ironic considering the politics behind oil today.
One of those who are mentioned to reside in Ptolomea, where are souls of those who killed their guests even though they aren't yet dead, is Branca D'Oria. Historically Branca died in 1325. while Dante died in 1321.
Ugolino committed treason, but you can't help but feel sorry for him after he's imprisoned in a tower to starve to death with his sons.
Francesca counts as well, particularly considering her sin (adultery) is far less heinous than Ugolino's. (Indeed, by modern standards she barely even qualifies for the "Jerkass" part.)
Magnificent Bastard: Ulysses, the famed hero of The Trojan Cycle, is encountered by Dante and Virgil within the realm of Bolgia. Ulysses helped to annihilate the city of Troy with his brilliant scheme of the Trojan Horse that brought the Greeks into the city. Having been lost at sea after, Ulysses won the hearts of his men with his subtle charm and manipulative wit, leading to his death in a bold attempt to explore further than any mortal man ever had before and accepting his resulting damnation with dignity and grace.
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.
Spiritual Licensee: Cartoon Network's Over the Garden Wall is the best-animated adaptation of the poem complete with similar characters (including a woman named Beatrice), story, and themes. Even the number of episodes and locations correspond with the Ten Circles of Hell. The similarities haven't gone unnoticed as multiple observers have pointed out how much Divine Comedy and Over The Garden Wall have in common.
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.
Among the heretics, Dante meets Cavalcante de' Cavalcanti, father of Guido Cavalcanti, who was Dante's friend-turned-political rival in real life. Upon meeting him, Cavalcante asks where is his son and, when Dante answers that Guido is not with him, believing his child is dead he cries out in despair and falls back into his fiery tomb.
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.
The people in Limbo, as they aren't even sinners but just lived in the wrong time and place (or didn't get baptized), and while they are not actively tormented, they still missed out on Heaven through no fault of their own. This is actually pointed out multiple times by Dante, who remarks in-universe how unfair this feels.
While Piccarda Donata went to Heaven, she is stuck on the lowest "level" (The Moon) for breaking her vows as a nun, even though she was forced into an Arranged Marriage and had no choice in the matter. While the poem says she's content with her fate, it seems rather unfair (particularly to modern readers) that she got put in the lowest level of Heaven for something that wasn't her fault.
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. And it is very controversial in some of the acts Dante considered as sins, but viewed by most modern-day people as matters of biology (like homosexuality) or mental illness (like hoarding and suicide); things that deserve compassion rather than condemnation. If Dante wrote today how these people should be tortured forever, he would be cancelled to the Tenth Circle himself, and bring endless accusations of God Is Evil (like in Larry Niven's Inferno). But it is important to note (as alluded to below) that the Comedy is an allegorical poem, not a theological work, and is therefore more an expression of Dante's thoughts and creativity than of the beliefs and teachings of Catholicism in the Middle Ages. Also, Modern Catholic dogma now subscribes to the doctrine of "Eternal Separation" (the suffering of Hell is mental, there is no torture but the despair and isolation of being deprived of God's love, which is closer to what Dante actually believed) and even says people who commit sins out of mental illness usually aren't culpable, as they aren't acting under their true free will.
Dante's whole hierarchy of sins with regards to the Circles of Hell seems rather odd at some places to the modern reader. Most of all, he considers fraud to be worse than genocide, as mass murderers are in Circle 7 and frauds are in Circle 8. However, once you understand the era where Dante lived in (the Feudal Era, the so called "Age Of Chivalry"), it becomes Fridge Brilliance. Every aspect of feudal society was based on oaths of fealty, of swearing Undying Loyalty to one's lord/benefactor/friend, and all oaths or contracts are sworn in the name of God. Thus, fraud, and especially treason, would probably be considered the worst kind of sins, as it is literally the destruction of societal order and sending a fuck you to God Himself. This leads to an unusual case of Values Dissonance within a case of Values Resonance. The Ninth Circle punishes treachery against kindred, country, guests, lords, and God. While non-religious readers and cultures may not care about the last one, the other four are almost universally reviled. The Dissonance comes from the order they're placed in. For instance, not many modern readers would consider betraying a guest worse than betraying your family.
The entire concept of Limbo. If you haven't committed any sins and lived before Christ was born, or instead an unbaptized child, it meant that you cannot access Heaven. Limbo was notably a controversial theological concept even in Dante's day and was largely undefined, giving Dante a significant amount of creative freedom in the way he depicted it. Certain decisions (making it a Circle of Hell, for example) are bound to read poorly to a modern audience.
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. Obviously historical appraisal of Caesar and his murderers varies tremendously with a person's political sympathies and how one views the political situation in Rome at the time. Positive portrayals of Caesar were especially common in Medieval Europe at the time (likely due to Nostalgia Goggles for the Roman Empire), waning significantly as republicanism started to catch on and the conspirators were viewed in a more positive light.
Satan's black face is unfavorably compared to that of an Ethiopian. If Dante lived in the modern first-world or in Ethiopia ever, he might have thought twice about writing that.
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 non-consenting victims could be 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. There is the additional fact that Piccarda's fate contradicts the actual teachings of the Fathers of the Church pre-dating Dante by almost a thousand years. St. Basil the Great and St. Gregory of Neocaesarea take care to explicitly state it in their rules that a rape does not make a woman less chaste.
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.
Dante also places clear limits on the spiritual powers of the Papacy: excommunication cannot damn a person who sincerely repents (though they will be in Purgatory for a very long time), the Pope cannot preemptively absolve someone of a sin he tells them to commit, and only sincere prayers of good people (not purchased indulgences) can speed up a souls progress through purgatory. The latter issue in particular would be a major element in the Reformation two centuries later.