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.
Complete Monster: We have the eponymous monstrous entity IT. Though an actual monster, IT is neither animalistic nor alien in its thought processes, it simply likes making people suffer. In the town of Derry, IT awakens from its hibernation every three decades and proceeds to murder and devour the children of the town, often using an avatar of a Monster Clown called Pennywise to lure them into its clutches. IT prides itself on using its shapeshifting and hallucinogenic powers to torment its victims, preying on their phobias and acquired fears and likening the cultivation of their terror to "salting the meat." IT was not only responsible for the disappearance of three hundred Derry Township settlers, IT also killed Bill's six-year-old brother, George, and devoured Patrick Hoffstetter alive while in the form of his greatest fear, leeches. IT then drove Henry Bowers to madness, and killed Bowers' friends after they succeeded in luring the Losers' Club into the sewers. After waking up in 1984, IT kills a man named Adrian Mellon before resuming its violent killing spree of children. IT proceeds to manipulate Henry Bowers into trying to kill the Losers; drives Bill's wife, Audra, catatonic by exposing her to its deadlights; and manages to kill Eddie before its final defeat.
Fridge Brilliance: One could argue that the whole concept of IT is sort of an Evil Counterpart to the Christian Trinity. There's the "Father" (IT's Giant Spider form), the "Son" (Pennywise, who is IT in "human" form), and the Unholy Spirit (the deadlights, IT's spiritual form).
Fridge Logic: Beverly deduces that the whole town of Derry knows something is amiss because an old man wouldn't help her. Note that this is only an issue with the movie.
Foe Yay: It's movie only, but the scene where Adult Ben thinks he's kissing Beverly when it turns out to be It in drag, who then shouts "Kiss me, fat boy!"
Genius Bonus: When the Losers go into the house on Neibolt Street to have their first confrontation with It as a group, Richie finds a bunch of rats with their tails tangled up together. This what is referred to as a rat king, and it is considered a very bad omen.
Ham and Cheese: While Tim Curry's performance as Pennywise, in the movie, is viewed as pretty scary by many, he also gives it a healthy dose of this, and it is ever so glorious.
Ho Yay: Henry and Patrick alone in the town dump...
In the movie, Eddie mentions that he could never be with anyone he didn't love, and he's never loved anyone but the rest of the Losers. In the book, Richie is the last person Eddie talks to, and Richie and kisses him goodbye after he dies.
Moral Event Horizon: It crosses the line when it kills and feasts on Bill's brother George Denbrough (who is only 6 years old), tearing his arm off.
Henry almost crosses this when he begins to carve his name into Ben's stomach with a switchblade, and eventually crossed it when he stole and poisoned Mike's dog and just For the Evulz and gleefully rubbed it in Mike's face.
Patrick crosses it when he smothers his baby brother.
Tom crosses it in the book when he beats Beverly's friend Kay nearly to death until she reveals where Beverly went.
Nightmare Retardant: Whenever Tim Curry's Pennywise goes over-the-top and silly. "Excuse me sir, do you have Prince Albert in a can? You do? Well ya better let the poor guy out! WAHA! WAHA! WAHA! WAHA!"
Slut Shaming: Many critics subject Beverly to this for having sex with the boys to keep them from freaking out after their first fight with It. The scene was only present in the book, but even movie-Bev gets hit by it. Many others, however, just object to the scene in general due to the character ages and situation. A more direct in-universe example happens when Mrs. Kaspbrak confronts the children, describing Bev's eyes as "decidedly slutty jade's eyes." If Bev had spoken, Mrs. Kasprak would have told her "what sort of girl ran with boys."
Special Effects Failure: IT's spider-form at the end of the movie. The true metaphysical nature of IT is only vaguely referenced. It looks very cheap and results in quite an Anti-Climax. To be fair, the cosmic elements of the novel would be tough to replicate on the screen. The Stop-Motion effects too.
Squick: In the novel, the gang, drained of energy after their first encounter with It in Its lair in the sewers, re-power themselves by losing their virginity with Bev. Is it emotionally significant to the story? Yes, but it's still a bunch of children having sex one after the other in a sewer.
There's also the scene where Beverly encounters an old woman who is IT in disguise. See Nightmare Fuel for details.
The Woobie: Every one of the Losers qualifies, with the possible exception of Richie.