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.
Fandom Nod: The pronunciation of "Hermione", which was finally clarified by Hermione herself in this book — then the films were released, so everyone knew how to pronounce it anyway.
Fridge Logic: Considering how most of the male cast looks in the film, it should be called "Harry Potter and the Year Nobody Got a Haircut".
"Funny Aneurysm" Moment: Ron notes at the end of "House Elf Liberation Front" that 'Percy wouldn't recognise a joke if it danced naked in front of him, wearing Dobby's tea cosy'. Cut to Deathly Hallows where Percy shows he does recognise and can tell a joke right before Fred is killed.
Dumbledore's remarks about Aberforth to Hagrid, notably his comment that he's "not entirely sure he can read", are amusing at the time. When, in DH, we learn about the estrangement of the two brothers, and the extent to which Albus' intelligence helped to contribute to that situation, it appears a considerably more mean-spirited thing to say.
Hilarious in Hindsight: After the First Task, when Madam Pomfrey, the school nurse, is attending to the Champions' wounds, she exclaims, "Dragons! ... Last year dementors, this year dragons, what are they going to bring into this school next?" Fans all know by now that the next year would see something far worse enter the grounds of Hogwarts School: Professor Umbridge as The Tyrant Taking The Helm.
The whole fact that Harry's incredibly handsome rival Hogwarts champion is played in the movie by future Edward Cullen. And really, the rivalry between Harry's supporters and Cedric's supporters could easily be compared to the Fandom Rivalry between Harry Potter and Twilight — particularly the way in which Harry and Cedric (a.k.a. Daniel Radcliffe and Robert Pattinson) are nothing but civil to each other while each has fans who volatilely bash their hero's rival. It doesn't help that Cedric first makes an appearance by jumping out of a tree. Maybe, he was bothered? His ghost is also sparkling.
"Can I have a look at Uranus too, Lavender?" You'll get your chance, Ron, when you start dating her in Book 6.
Ron asked his future sister-in-law to the Yule Ball.
It's now almost impossible to watch the reaping... er, choosing of the teenaged contestants in a deadly tournament without thinking, "May the odds be ever in your favor" as their names are called out.
Idiot Plot: The whole plot of the book hinges on the fact that Bartemius Crouch Jr. was too dumb to just use a normal object as a portkey instead of the Triwizard Cup to transport Harry to Little Hangleton graveyard. And for having Harry use said portkey in the most publicized event in Hogwarts history, as opposed to the privacy of his own office.
The justification was that Voldemort was trying to make Harry's death look like an accident. In a highly dangerous tournament, that'd be easy to pass off. Even if they guess anyway that it's a murder, due to Harry being somehow put in the tournament against his will, they wouldn't be able to tell that Voldemort did it.
Launcher of a Thousand Ships: Continuing the series tradition, the introduction of new characters as well as already existing ones being made prominent created some new ships, including Harry/Cho, Cedric/Cho, Harry/Fleur, Ron/Fleur, Hermione/Krum, Hermione/Cedric, Krum/Fleur, Cedric/Fleur.
From the film. Harry's massive putdown to Malfoy? "Your father is vile and cruel! And you're just pathetic." It's that last part that somehow wounds Malfoy deeply enough to try and curse him.
Also David Tennant's weird tongue flick thing as Barty Crouch Jr. It's meant to be creepy and yet comes across as pantomime villain stuff.
Another one from the film - Hermione breaking down in tears at the steps after Ron "spoils" her big night at the Yule Ball - well actually the whole darn Yule Ball argument, what little of it is there. While the original chapter always carried a hint of satire on teen angst, it comes off looking even more ridiculous in the film due to the rather lackluster execution of the scene, with Rupert Grint (Ron) delivering his lines - which in the book were written with considerably vicious jealousy - with about as much interest as someone ordering a hamburger. Meanwhile Emma Watson (Hermione) starts off equally understated to the point of being just plain flat, and then she goes for the Large Ham approach with her reaction, which seems disproportionate to Grint's lack of investment in the scene. That her having the last word in the film actually gets undermined and upstaged by Ron doesn't help.
And before the rather lackluster argument between Ron and Hermione there's Hermione's rather underwhelming Beautiful All Along debut at the ball - she pokes her head out shyly from behind the corner, slowly emerges (her Yule Ball dress changed from the periwinkle blue of the book to an unflattering shade of pink, probably to stand out against the blue background), and then we see her descend from the top of an unnaturally tall windy staircase as if she were a Disney princess (and the scene does feel rather reminiscent of the end of 1959's Sleeping Beauty) and Harry smiles in surprise as she descends before Ron gets a look at her. It's supposed to be really breathtaking, but despite all the fancy lighting tricks and sickeningly sweet fairy tale music, like so many similar scenes in big splashy Hollywood films before and after it the scene just falls horribly flat due mostly to the fact that Emma Watson is just too naturally cute for it to work - never mind that none of the filmmakers ever did a particularly effective job of trying to make her look plain or ordinary before this film. It really does feel like it's just the same girl from all the previous scenes (and previous films) in a dress with a slightly different hairstyle; even Daniel Radcliffe (Harry's actor) lampshaded it in an interview. The fact that Hermione's altered dress makes her look like Princess Bubble-Yum just makes the scene (and her part in the Yule Ball) look and feel even more ridiculous.
Not to mention the "HARRYDIDYEPUTYERNAMEINTHEGOBLETOFFIYAH???!!!?" moment from the film, especially comparing it to the way Dumbledore calmly asks it in the book.
The entrance of the Beauxbatons students; they run up the centre aisle of the Great Hall and then...basically seem to fart bluebirds whilst sighing happily.
Nightmare Fuel: Rowling was apparently surprised that the editors didn't object to Voldemort's resurrection scene, specifically Voldemort's fetus-like rudimentary body.
Wormtail cutting off his own hand.
The trial of Barty Crouch Jr, with Junior begging and crying for mercy and Senior showing no emotion or connection towards his son until he finally loses it and screams at him that he has no son. YMMV, since at the end it's revealed that Crouch Jr. actually was evil and guilty, though his story still is tragic. Also, the torture of Frank and Alice Longbottom.
For those with a fear of deep, dark water, the Second Task as a whole.
No Such Thing as Bad Publicity: This entry brought the Moral Guardians out in full force against the series, ensuring this trope occurred. While still a good book in its own right, it wouldn't have been nearly as successful without all the parents, politicians, and preachers decrying it for converting children to witchraft and Satanism. Some held burnings of it. This required them to buy copies to burn them.
Ship Sinking: Cedric and Cho. Despite this book launching their ship, it's torpedoed at the end when Cedric is killed.
Ship-to-Ship Combat: Also, continuing the series tradition, the supporters of the new and already established ships took all new offensives for their pairings.
They Just Didn't Care: As seen in Narm the Yule Ball argument in the film is too flat to be either funny or dramatic and is handled with such carelessness that it might as well have been left out altogether.
Why Don't You Just Shoot Him?: The story would've been much shorter, had Voldemort stabbed Harry with the knife, rather than challenging him to a duel, thereby allowing him to escape. Justified, though; Voldermort was too egotistical to simply kill his archenemy in such a mundane, Muggle-like way, and was also trying to prove that he actually was more than a match for Harry and that his "death" was just a fluke, which admittedly it was. Harry didn't even know any good combat spells and once again only survived because of luck.
The Woobie —> Jerkass Woobie: Barty Crouch, Jr. When we first see him, he is pleading for mercy as his own father disowns him in public and orders him thrown into Azkaban. Unfortunately, it then turns out his father may have been right, as Crouch Jr. turns out to be a fanatical Death Eater who tortured Neville's parents into insanity, not Wrongly Accused as assumed.