These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Black and Black Morality: One of the many alternative character interpretations. God is an absolute dictator, Satan is a jerk who wants to ruin the plan out of spite, and humanity is like the kids caught in the crossfire of a nasty divorce.
Draco in Leather Pants: Satan's conflicted, stirring speeches and soliloquies are some of the best writing on the book, causing him to have a legion of fans. William Blake was probably the first to suggest that Milton was subconsciously on his side, and Satan has developed one of the most loyal Alternative Character Interpretations in literature.
Evil Is Sexy: Lucifer is indeed the most beautiful of all the angels, and even as Satan possesses a great deal of charisma and a seductive, salacious tongue.
First Installment Wins: Paradise Regained, the sequel, is about the Devil's temptation of Christ. It's shorter and not nearly as frequently read or critically acclaimed.
Funny Moment: Following Adam's & Eve's celebratory sex after eating the fruit, Adam blames Eve for their current predicament, and they end up not speaking to each other for the rest of the day.
Moment Of Awesome: Gabriel facing down Satan in the garden, Michael cutting Satan's legs off, the angels responding to the rebels' war machine by dropping a mountain on it, Abdiel's "Kirk Summation" speech in Book 5, and finally, the Son smiting all of Satan's forces single-handedly, causing the demons to throw themselves into Hell.
Nightmare Fuel: Satan gave birth to Sin out of his head; she was, at least to Satan, beautiful enough to have sex with; after they were all thrown into Hell, she gave birth to Satan's child, Death; he, in turn, ran after and raped his mother; the resulting birth tore up Sin's lower parts so badly that her legs are now a snaky tail, which is not surprising because the babies were all Hell Hounds. The beasts continually chase around her, barking and snapping at each other, and regularly crawl back into her womb, gnawing on her intestines from the inside, then claw their way back out.
Squick: Satan gets his daughter pregnant, and then their son rapes her. The resulting children, Hell Hounds, crawl back inside her when approached, gnawing at her entrails from inside..
Unintentionally Sympathetic: John Milton’s intentionally subverted this trope. Since the poem starts off from his viewpoint, Satan comes across as more of a Designated Villain than an actual bad guy. However, as the poem progresses, the reader is shown the story from the point of view of the angels and God, and it becomes clear that Satan is rationalizing his behavior just like humans tend to do. The reader is supposed to sympathize with Satan, but they are not supposed to realize why they are doing so until God (literally) tells them why he is wrong. However, because of the eloquence of his passionate arguments, even many who have read the work miss the point, and so believe that “Satan” is in fact the hero of the story, making this a straight trope. This may be a case of Values Dissonance mixed with Cool People Rebel Against Authority.
Values Dissonance: "What?" you say. "Seventeenth century religious fiction features Values Dissonance? No way!"
Word of Dante: If there's a portrayal of Hell and/or Satan in fiction that isn't based on The Divine Comedy, then it's likely taking cues from Paradise Lost. The same goes for Heaven (Milton likely canonized the idea of angels playing harps).