Laser-Guided Amnesia: Type 3: Something (implied to be the Other) makes them forget about each other and their childhoods after they leave Derry, with only Mike remembering because he never left.
Victory Guided Amnesia: Except for Mike, all of the children completely forget their victory and spend most of the adults' portion of the novel trying to remember. After their second victory, they all begin to forget again.
Scars Are Forever: Zig-zagged. Henry cut an H into Ben's belly when they were kids, and a much deeper scar was inflicted on him a little later. The H scar stayed (and became a neat pub story) but the deeper scar faded away after they left Derry. When IT resurfaced and the Losers started being drawn back to Derry, the scar reappeared.
Domestic Abuser: Beverly's husband Tom. In her first scene, she gets the call from Mike, and when Tom tries to stop her from leaving, she decides she's had enough of his shit. They fight and Tom gets the worst of it.
Hello, Nurse!: It is acknowledged in-universe that Beverly is very beautiful. The reason that she isn't part of the popular girl's clique is that she's poor and can't afford nice clothes, and she doesn't act very ladylike (swearing, smoking, playing out in the woods instead of doing stereotypically girly things, and hanging out pretty much exclusively with boys). Also, the richer girls are rather annoyed that they are getting upstaged in terms of looks by someone who's working-class and from the poor side of town.
Heroes Want Redheads: All the male Losers are romantically attracted to Beverly at some point, as is Beverly herself to Bill and they all have sex with her - although Ben becomes a couple with her at the end.
Butt Monkey: He grows up as a fragile, allergic-to-everything Momma's Boy, gets his arm broken by Henry and pals, and marries an overbearing carbon copy of his mother.
It's even worse in the movie, where he never stops living with his mom and dies a 40-year-old virgin.
Calling The Old Lady Out: Eddie is pissed when he's in the hospital and his mother sends his friends away. She is actually frightened of him for a moment. Note that this is the only time he ever stands up to his mother.
Arch-Enemy: To Henry Bowers. Henry hated Mike the most out of all the Losers. He learned it from his father Butch, who absolutely detests Mike's father Will, mainly for the fact that he's black, but also because he's simply a better farmer.
I Choose to Stay: He never left Derry even after the rest of the Losers' Club had moved, and essentially acted as the watchman in case It ever returned.
Though he didn't precisely "choose" to stay any more than the others "chose" to leave. They left because their parents moved away, he stayed because his never did. It was less choice than it was fate and possibly the manipulation of the Turtle.
Informed Judaism: In the book, Stan tells his friends his family are secular Jews, and do things like eat ham and work on Saturdays.
Skepticism Failure: Stan is the last of The Losers' Club members to recognize IT's existence. There is the implication that his extremely ordered, rational nature is what led him to choose suicide rather than face It again, a monster that defies rationality and natural laws.
Achilles' Heel: It has to take a physical form in order to directly influence the world, however this means that It can be hurt and banished (though it's not clear whether It can actually be killed in this manner.
Ax-Crazy: Is cold, cruel, loves to kill, and is utterly beyond human comprehension.
Big Bad: Is the primary antagonist of the novel and movie.
Blob Monster: When IT emerges from a drain to attack Beverly, she notes IT is a shapeless mass with a taffy-like consistency.
Child Eater: IT prefers to munch on children because their imaginations and emotions are more vivid (read: juicy).
Dirty Coward: IT's brazen enough when she's in control of a situation, but begs and pleads as soon as she realises she's vulnerable, and her last words (apart from a Big "NO!") are frantic attempts to bargain for her life.
Giant Spider: Not IT's true form, but probably as close to it as any human is able to perceive. It's more of an "anchor" that allows It to exist and influence the world.
Gender Bender: IT's favourite form is Pennywise the Dancing Clown, which is implicitly male, but IT is actually female.
Genius Loci: IT practically is Derry. The fact that Derry recovers following the end of the book is a hint that IT is still alive.
Hoist by His Own Petard: IT shares the weaknesses of whatever form she takes. Also, if several people all perceive IT as one form and think of her in that form hard enough, she becomes "mode-locked" and unable to change. The Losers take advantage of this in the house on Niebolt street. They first lock it into werewolf form, then they drive it off by shooting it with a silver bullet from a slingshot.
Load-Bearing Boss: After IT is finally defeated, a flood destroys much of Derry later that year. It's implied that IT had allowed the town to exist in exchange for providing her victims every 27 years.
Mama Bear: Not exactly a trope one would associate with a child-eating eldritch abomination with a pom-pom fetish, but when the Losers start smashing her eggs and killing her spawn after making her turn tail, she comes back with a vengeance.
Reality Warper: If one believes enough in IT's illusions they become real.
Red Oni, Blue Oni: Pennywise/the Spider and the Turtle. One actively hunts down and eats human children while the other just sits on the edge of forever, seeing it all happen and "helping" the Losers during their first confrontation with IT. The Spider berates it for just sitting there, offering seemingly useless advice. That the Spider's eyes are described as ruby-red while the Turtle's shell is some blueish-greenish color also reinforces the trope. IT suspects/fears an "Other" beyond the Turtle that is also opposing IT, but it's not until The Dark Tower novels that this is confirmed.
Shapeshifter Default Form: Pennywise the Dancing Clown is a Type B, being the form IT uses to get around and interact with people.
Shapeshifter Mode Lock: Happens to IT when the entire Losers' Club perceives her as a giant eyeball. Notably, one of the characters was about to perceive her as something else, but when one of them shouted "It's a giant eye!" she appeared to all of them in that form. Also happens earlier when they perceive IT as a werewolf, and shoot IT with a silver slingshot bullet.
Smug Snake: IT is arrogant and sadistic when in control of a situation, but when the tables are turned she retreats.
Will Hanlon: The father's a turd and the son's a little fart.
Mean Character, Nice Actor: Jarred Blancard as Young Henry Bowers. According to the DVD commentary he felt really bad for having to use the N word and couldn't be more apologetic to Marlon Taylor (Young Mike) after they finished filming the scene.
Politically Incorrect Villain: He hates Stanley because he's Jewish, he hates Mike because he's black, he hates Eddie because of his asthma, he hates Beverly because she's a girl, he hates Ben because of his weight, he hates Richie because he's a little smartass four-eyes twerp, he hates Bill because he's a stuttering nerd, we can go on forever.
Schoolyard Bully All Grown Up: After spending many years in psychiatric hospital, Henry Bowers escapes with IT's help and almost kills Mike Hanlon.
Teeny Weenie: Compared to Victor, as Beverly notices when she watches them light farts from a hiding place.
The Sociopath: He's contrasted with his two primary associates in bullying, Victor Criss and Belch Huggins, in that Victor and Belch like picking on the other kids, even beating them up, but they don't want to do any lasting harm. Once he gets angry enough, Henry simply doesn't give a fuck about the consequences.
Even Evil Has Standards: Victor is fine with beating up smaller kids for fun, but shocked by some of Henry's actions (such as trying to carve his name on Ben's stomach with a knife).
Gag Penis: Beverly makes note of it when she sees Henry's gang lighting farts from a hiding place. Contrary to the trope name, it is NOT played for laughs.
Heel Face Door Slam: There are some hints in the book that Victor was considering defecting to the Losers, and may have gone as far as warning them about Henry's deteriorating state. If he was considering this, though, IT put a stop to that.
Noble Demon: Not a completely straight example, but he has some shades of it. He has no problem beating other kids up, but, he will never go as far as to do permanent damage to them. While being chased by Henry's gang before the Rock Fight, even Mike acknowledges that out of all of them, at least Victor doesn't want to do him any serious harm.
Only Sane Man: Victor is among the first to realize just how far off the deep end Henry is going, and likely the first to do so on the bad side.
Red Oni, Blue Oni: Although never acknowledged as equals, he is the Blue to Henry's Red. This is especially clear in the Rock Fight. Henry got hotheaded, and only got himself hurt even further. Victor kept his cool, and was able to take all the damage Henry's Gang suffered, and return it in kind back to the Losers by himself.
Even Evil Has Standards: Beating up smaller kids for fun is fine by him, but he is pretty disturbed when Henry tries to carve his name on Ben's stomach.
Heroic Sacrifice: Even though he's been a bullying Jerk Ass for most of the book, Belch actually redeems himself when he protects Henry from IT after the monster kills Victor and goes after Henry. Unfortunately for him, he gets half of his face torn off for his trouble.
Creepy Child: The kid collects dead flies. That is, flies he swats with a ruler and puts into his pencil case. And that's the least creepy thing about him.
Depraved Bisexual: Fondles the girls in his class, masturbates Henry and offers him oral sex.
Fearless Fool: because of his mental state, Patrick has little understanding of the concept of fear. Because of this, IT is not quite sure what form (besides the flying leeches) to take when it attacks Patrick (Patrick notices the thing approaching him is constantly changing shape, as if not sure who or what it wants to be).
Karma Houdini: Discussed and ultimately averted. When his younger brother was born and Patrick lost some of his parent's attention, he smothers the baby with a pillow. His father did become suspicious at one point, but he ultimately decided not to take action against him. No one ever finds out, but Patrick eventually meets a grisly fate courtesy of It.
Karmic Death: Is eaten alive by It in the form of a swarm of leeches.