Why did the Ritual of Chüd fail to stop IT (twice no less) despite the fact the first phase seems to work fine? Mere superstition that IT pretends to play along to crush their hopes? Or is it due to the fact that the ritual still treats IT as a feared Eldritch Abomination rather than something small and weak? IT is ultimately affected by how people perceived it, as demonstrated by the Losers, and if it's still perceived as anything but weak in the past, then of course the ritual would fail to contain IT every time it's performed.
On those same lines, it's the reason why the iron rod actually hurts IT, as opposed to the Ritual. The iron rod is a simple weapon meant to kill ordinary creatures of flesh and blood. By throwing the rod at Pennywise, Eddie treats IT as a mortal being that can be hurt, and that perception translates into reality.
Also, in some folk tales, the devil is allergic to iron.
Additionally, what if they misunderstood how the ritual was supposed to work? What if it required not only destroying the tokens of their trauma but also the trauma itself? They went through the motions but didn't conquer their fear so IT was largely unaffected.
Although it just seems like Pennywise is just being his usual cruel self when traps Richie in the deadlights during his verbal rant at him, it was probably his own defense mechanism as the Losers giving him a "The Reason You Suck" Speech is what ultimately does him in.
The 'Angel Of The Morning' part was agreeably goofy... but consider, was that song playing just in the movie as an editing choice? Or was that playing in Eddie's head too? They point out that Eddie was moments away from actually killing IT, maybe this was just a random move to throw him off as much as possible. Once it's over, IT disappears altogether, as if he's running and hiding to recover.
Why does Pennywise allow the Losers to band together in its lair and perform the Ritual of Chüd, when he has plenty of prior opportunities to pick them all off one at a time? It's because A) he knows the Ritual won't harm him, and B) he wants to affirm his belief in his invincibility by killing the Losers while they're together.
Beverly reveals that she saw the Loser Club's deaths when she was caught by the deadlights during the group's encounter with Pennywise in their first cycle. She explains that they won't live to see a third cycle, because when they were children, Pennywise messed with something in their heads, and none of them will make it another 27 years without killing themselves. It really seems like Pennywise attempted to cover all his bases: either the Losers return to Derry and he kills them, or they stay as far away as possible and kill themselves. Of course, neither of these came to pass in their entirety, but it was almost successful effort for Pennywise.
The jump-scares in the movie last from the beginning right up until Pennywise's death... because Pennywise was responsible for them. When he dies, there's no source for the scares anymore
Kinda obvious point here
After Stan is told by Mike that It has returned, Stan ends up committing suicide because he is too afraid to go back to Derry and face Pennywise with the rest of the Losers. Before he does so, he remembers the promise he made to Bill that he would be there to help them if It came back, even saying, "I swear, Bill, as he lays in the bathtub. Kind of a strange thing for him to say in that situation, right? Well, this makes perfect sense after The Reveal at the end, when the rest of the Losers learn that Stan decided to kill himself to give the Losers a better chance of destroying It, as he figured that his fear would hold the group back and possibly get them all killed. When he said, "I swear, Bill", he really was promising to be there for the Losers, just in spirit rather than in person.
During the "I know your secret" sequence, Pennywise starts doing his iconic dance towards Richie, but it's a little more... exaggerated than normal. Think back to when the Losers were in the clubhouse together... and Richie started doing the Pennywise dance. Pennywise is telling him 'I know you made fun of me' just to drive the terror home
The over-the-top CGI monsters such as the lumberjack statue and the Mrs. Kersh monster, as well as Pennywise's half-clown, half-spider form may seem Narm-y to some, but it makes sense as this is Pennywise we're dealing with. He thrives on children's fears, so naturally his methods of scaring and attacking victims would be more jumpscares and monsterous creatures that would terrify any child. He's pretty much a boogeyman in cosmic Lovecraftian horror form.
Richie always told I Banged Your Mom jokes to Eddie as a way to not only tease him but to show affection as an Armored Closet Gay. So when Eddie's Famous Last Words were this to Richie, it could've been his way of saying "I know how you feel about me Richie, and I feel the same way towards you".
The fact that Eddies body is buried underneath Neibolt and unrecoverable. Not only does this deny Richie any closure, but who gets to explain what happened to his wife?
To be fair, they never got Eddie's body out of the sewers in the novel as well.
On the topic of Eddie's wife, what about her? Her husband's now dead. Even if it wasn't the happiest or most stress-free marriage, and even if she wasn't the nicest wife, she probably still loved him and would be devastated when the word comes out. Plus, it seemed like Eddie was the one who payed the bills. What's going to happen to her financially?
Also, Dean and Victoria's family. Assuming they aren't abusive, or completely neglectful, they are probably having extreme guilt over not looking after their children, and allowing them to get devoured by Pennywise.
For that matter, Don too. The last he ever saw of his boyfriend was suffering a violent homophobic attack, and then a brutal death under the hands of Pennywise. That's the only scene we ever see of him and we never find out how he's doing.
Pretty much all of Pennywise's other victims, too. We don't know much about them, but we do know that they were still people, mainly children, with futures, loved ones, and lives that were cut short because of this clown. The Losers may have destroyed Pennywise, but that won't undo all the damage he's done, all the victim's he's claimed, and all the grief he's stricken to families.
In the novel, Beverly's abusive husband Tom Rogan follows her to Derry and is killed by IT, but this was left out of the movie, meaning she was still legally married to him at the end.
It's all but confirmed that there was, in fact, a real life man named Bob Gray that IT based the Pennywise form off of. The same form that IT more or less maintains as often as possible, even as IT dies. Given that IT seems obsessed with the form, just what did IT wind up doing to Bob Gray after getting so obsessed? Do we even want to know? And for that matter, if Bob Gray had a family like it was implied with the Mrs. Kersh scare that Pennywise pulled on Bev, what happened to them.