"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.
At one point in the book, Fred and George tell their angry mother that if she yells at them now and the Hogwarts Express crashes tomorrow, that'll be the last thing she ever said to them. In context, it's pure Black Comedy (even to Molly), but a year later we find out that Molly's biggest fear is losing her family. Worse in Deathly Hallowssee above spoiler.
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.
Reread this particular book after reading the last one or watching the last movie. Seen it yet? Well, note Fred and George's failing potion to age them. This is the only time the two will grow old together...
Genius Bonus: The Durmstrang Institute's name is a spoonerism of the German phrase Sturm und Drang, the name of an 18th century Proto-Romantic literary movement known for its glorification of subjective human emotion and experience over the cold rationalism that characterized the Enlightenment. It's a fitting name for a Scandinavian school for wizards, which teaches Magic that that can't necessarily be understood rationally. Particularly since the most famous Sturm und Drang participant was Johann Wolfgang von Goethe, the author of Faust—a play about a German mystical scholar who dabbles in the Dark Arts in his quest for knowledge.
Both Harry and Cedric are initially outraged that the Quidditch pitch has been turned into a maze for the third task. Bagman reassures them that they'll have their pitch back once the task is over. What's harsh is that Cedric never gets to see the Quidditch pitch back to normal, since he is killed by Voldemort and Wormtail during the Task.
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.
Overlapping with Harsher in Hindsight: Filch putting on music as Professor Mc Gonagall dances with Ron can remind many of Walder Frey (portrayed by David Bradley) ordering to play "The Rains of Castamere" during the Red Wedding.
The fans' uproar over Hermione wearing pink instead of blue to the Yule Ball. Fast forward to Ballet Shoes where a small fuss is made over Pauline (Emma's character) getting a blue gown for a premiere. So fans finally get to see Emma wearing blue at a formal occasion.
Idiot Ball: A common criticism of the villain's plot in this book is 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. And, since he can apparently enter Harry into a Binding Magical Contract against his will, not entering him into one which compels him to take a walk off a cliff.
Idiot Plot: The only reason given for Harry participating is to bait out whoever put his name in the Goblet; he wasn't interested in participating even if it wasn't age restricted. The villains' plot relied on everyone playing along, and would have been thwarted completely if Harry did nothing. And yet none of the staff of Hogwarts or Harry's friends (ostensibly both interested in his welfare) suggest he do anything other than walk into what is an incredibly obvious trap.
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.
Magnificent Bastard: Voldemort and Barty Crouch Jr. With the information coerced from Bertha Jorkins about a month before the actual event, Voldemort was able to concoct a plan to not only seize the Harry Potter's blood and use it for his revival, but recruit a Death Eater that was long since believed dead. Barty Crouch Jr. not only successfully escorted a Book Dumb 14-year-old through a dangerous tournament designed for 17-year-olds, but also did so without drawing any suspicion to himself, disguised as the Death-Eater-hating Auror himself. Not only that, but even though Harry escaped Little Hangleton, the plan Voldemort came up with made sure that the wider wizarding world didn't believe his comeback until a year later: not the success he envisioned, but still incredibly useful.
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.
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.
Cedric's father after the death of his son.
Harry Potter, too, for the same reason.
Harry gets hit with this earlier in the book: After being chosen as Champion his life quickly goes to crap: The other three houses turn against him, believing he put his name in to snatch more glory for himself and even Ron buys into this. This image isn't help by Rita Skeeter's article glorifying Harry using mostly made up garbage, which causes Harry to snap at his crush Cho and only drives a further wedge between him and Ron. Oh and on top of all of this he has to fight a ferocious dragon with only a day to prepare. All for an honor he didn't want and would gladly give up if he could.
He Really Can Act: Look at Daniel Radcliffe's crying in Prisoner of Azkaban after The Reveal that Sirius is his godfather, which is just Narm-y. But Harry's complete breakdown after he arrives back at Hogwarts with Cedric's corpse along with the news that Voldemort has returned? One of the biggest Tear Jerkers in the film series.
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.
Hermione breaking down in tears at the steps after Ron "spoils" her big night at the Yule Ball - or in fact, actually the whole 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. Rupert Grint (Ron) delivers 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 fairly 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.
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.
Dumbledore's speech at the end is filled with cliches in the style of "you all knew him" and Damned by Faint Praise in regards to Cedric. This makes it sound hollow and insincere, especially in comparison to the more solemn but realistic speech in the book.
They Changed It, Now It Sucks: Showing how some fans can take this trope to the extreme, one of the biggest complaints about the movie? Hermione's dress for the Yule Ball. It's blue in the book but pink in the film. There were thousands of complaints, even with the film's IMDB page in an edit war over listing "Hermione is a person who hates pink and would never wear a pink dress" as a Plot Hole. note The costume designer said that blue was the original choice but that it simply didn't look good on Emma Watson. Here◊ is an edit of Hermione in the blue dress for reference. It's worth noting that the rest of them didn't wear the colours they had in the book for the film either - Harry went from green to black, Parvati went from pink to orange and Padma went from turquoise to pink. As for Hermione "hating pink", she's mentioned as wearing a pink dressing gown in the first book.