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.
YMMV: Harry Potter and the Goblet of Fire
Abandon Shipping: Cedric and Cho. Despite this book launching their ship, it's torpedoed at the end when Cedric is killed.
"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 and immediately after Fred is killed.
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.
Also, David Tennant said in an interview at the movie premiere that he'd probably never be part of such a large fandom again. Oneyear later...
"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.
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.
And Voldemort needed Harry at the Graveyard to get a body.
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 that somehow wounds Malfoy deeply enough to try and curse him.
Also David Tenant'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. 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, while Emma Watson (Hermione) 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.
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-to-Ship Combat: Also, continuing the series tradition, the supporters of the new and already established ships took all new offensives for their pairings.
Why Don't You Just Shoot Him?: The story would've been much shorter, had Voldemort stab Harry with the knife. Rather than challenging him to a duel, thereby allowing him to escape.