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.
Luna, in the mistletoe scene in the Room of Requirement: Legitimately worried about Nargles, or covering for her dashed hopes that she could finally snag a kiss from someone?
Because of how little characterization she had, Marietta Edgecomb is subjected to this. Was Cho telling the truth that she only ratted out the DA because she didn't want her mother to lose her job, or was Cho lying in order to cover her ass?
Angst Dissonance: Harry's angst after Cedric's death and being left out of the fight against Voldemort by the Order (because the kid who saved their asses four times already can't possibly be useful), while justified, was still an annoyance to many readers.
It's likely that Harry has PTSD in this book. He watched a classmate die, was tortured, and forced to fight an older wizard with over 40 years and a lot of power on him. His angst is perfectly reasonable, but according to some doesn't make it any less obnoxious.
Anti-Sue: Umbridge is given the inverse of many Common Mary Sue Traits, is hated in-universe, and completely derails the plot set up by the previous book while simultaneously diluting the plot of this book.
Big Lipped Alligator Moment: Subverted with "Snape's Worst Memory". All we get after seeing it is some angsting from Harry over how big of a jerk his father and Sirius were as teens, and then it's never mentioned again. However, in Deathly Hallows, we find out precisely why it's Snape's worst memory: It destroyed his friendship with Lily, the only woman he ever loved.
Crowning Moment Of Awesome: Many, such as when Harry is ambushed by Slytherins who are angry about their fathers being sent to Azkaban during the ending. They unwisely choose to attack him outside of a compartment filled with DA members, who all rush out and jinx the Slytherins so thoroughly that they apparently resemble slugs.
Freud Was Right: Dolores Umbridge's "avoidance and theoretics" approach to D.A.D.A as opposed to actual knowledge drew a lot of parallels to the U.S. sex ed debate. And that book was heavy handed as it was.
The Room of Requirement: a hidden area where teenagers sneak off to engage in "forbidden activities" and risk getting into deep trouble if caught.
"Funny Aneurysm" Moment: It's too easy to laugh at Neville's constant screw-ups, plus the fact that he lives with a near-tyrannical grandmother. But then in the fourth and fifth books, we learn exactly what happened to his mother and father, and how it has affected him. Then we feel guilty for having laughed.
Genius Bonus: In St Mungo's, Lockhart is wearing a lilac-coloured robe. Three books ago, he said that lilac was his favourite colour — a hint that some of the old Gilderoy remains.
Growing the Beard: This did it for the remaining critics, if POA and GOF failed to do so.
Zacharias Smith's "I don't think Expelliarmus is exactly going to help us against You-Know-Who, do you?", given that this spell is what finally brings Voldemort down.
Harry says sarcastically to Malfoy, Crabbe and Goyle: "I'm terrified now. I suppose Lord Voldemort's just a warm-up act compared to you three?" In Deathly Hallows, those three are a warm-up act before the final confrontation with Voldy.
Launcher of a Thousand Ships: Since this introduced some new characters, it also launched a few new ships. Most prominent was Harry/Luna and Neville/Luna.
Love to Hate: Few, if any, fictional characters are as supremely and utterly despicable as Dolores Umbridge.
Memetic Mutation: Umbridge's "I will have order!" She says it once in the movie and never in the book. Tellingly, by the time the next film was made, they saw fit to include an Umbridge doll in Weasleys' Wizard Wheezes which constantly squeals "I will have order!"
Moral Event Horizon: Dolores Jane Umbridge has long since passed over it. She starts by cheerfully calling Harry a liar, forcing him to cut into his own flesh and do lines with his own blood, tries to sabotage Gryffindor's chances of winning the Quidditch Cup by removing three of their best players, cruelly mocks Hagrid's teaching, gleefully tries to sack Professor Trelawney (and thoroughly enjoys her despondency), takes over from Dumbledore, tries to arrest Hagrid for no reason, nearly kills McGonagall in a sneak attack, and finally tries to used the Cruciatus Curse on Harry. Oh, and it turns out she was the one who set Dementors on Harry and Dudley in the beginning. So, take your pick. Oh, and guess what? She gets worse in Deathly Hallows.
In the movie, there's a scene that shows that she forces first years to use the Blood Quill.
UncleVernonDursley crosses it in the novel (but not in the film) when he tries to kick Harry out of the house, after finding that he would likely die in the hands of Lord Voldemort if he did this.
In his very slight defence, his reasoning for doing so are for the sake of his wife and son and their safety, not specifically because he wants to see Harry killed (though he wouldn't likely care if his nephew was killed). Vernon, not understanding magic and that he and Harry are both better off with Harry there, thinks he'd be protecting his family by getting rid of Harry.
Dudley certainly qualifies for a moment at the beginning of the film where he becomes especially cruel to Harry by mocking him with the things he says in his sleep.
Dudley: "No, not Cedric." Who's Cedric? Your boyfriend? "No, don't let them get me, Mum." Where is your mum?...Is she dead?
Never Live It Down: The release of the fifth book and the "Snape's Worst Memory" chapter has become the defining image of James Potter's character among the book's fans and especially Snape's fans, rather than the positive way he was regarded and described in the first four books as a great wizard, loyal friend and good father.
Nightmare Fuel: The Death Eater whose head — just his head — was regressed to infancy.
When those brain things attack Ron in the Department of Mysteries.
Umbridge. She sits in as a voting member of people's trials? She's allowed around children?! Her tactics are so draconian she may as well be a Death Eater. And Harry says so himself: "She's foul enough to be one."
The bureaucracy that is the Ministry Of Magic for turning a blind eye to the atrocities Umbridge committed. The Death Eaters had taken over the Ministry when Umbridge was at her very worst but this in no way excuses her actions.
Stop Helping Me!!: Even though Harry wholeheartedly appreciates their courage later and they eventually become more competent in later books, itís rather hard to say exactly how much use his friends really were during the Department of Mysteries. Neville and Luna probably stumble the hardest, as the former accidentally disarms Harry during a fight and later unintentionally lands himself over as a hostage to force Harry into surrender (after Harry had bought them an opening to flee), and the latter breaks Ginnyís ankle trying to help her, effectively knocking one of the DA's best fighters out of the battle.
The Scrappy: Umbridge is actually hated by some fans even more than the Big Bad. In this case, it's actually perfectly justified.
In this case, it was definitely invoked. She's unattractive, so as not to create Draco in Leather Pants, and she has absolutely no Pet the Dog moments, to avoid Alternate Character Interpretation. Plus, a person with power over you, who uses that power as much as possible and loves every minute while showing as much self-righteousness as they can muster...admit it, you knew or still know someone like that, and you hated them with every fiber of your being.
Zacharias Smith of Hufflepuff is depicted very unsympathetically for merely asking why everybody is accepting Harry's story that Voldemort is back. Keep in mind that Harry's been described by the adult authorities as "disturbed and dangerous" and as an attention-seeking glory-hound. Also, Zach might have other reasons to distrust Harry—-Harry told a tabloid newspaper about Cedric Diggory's death first, instead of the boy's housemates, who idolized him and really wanted to know. For this high crime, Zach has become the fandom's Butt Monkey; very few characters get less sympathetic treatment than he does.
Marietta Edgecombe betrays the DA to Umbridge. This activates a secret jinx from Hermione that writes "SNEAK" across her face in boils. Cho considers this a mean trick, and eventually breaks up with Harry over it. Thing is, Marietta was motivated by the risk of her mother losing her job at the Ministry, never wanted to join in the first place (Cho pressured her), and it's kind of unethical to trick people into signing a contract that punishes them for betrayal without telling them. (Heck, Hermione didn't even let her best friends know what would happen until later.) Rowling herself seems to be firmly on Hermione's side; when she confirmed that Marietta was permanently scarred, she said "I loathe a traitor!"
Interestingly, Dolores Umbridge has the same opinion. She won't stand for disloyalty to the Minister for Magic, going to extremes (including physical abuse) to express this viewpoint. She permanently scars Harry with the words "I must not tell lies" on his hand. Food for thought.
The Ministry's claim that they need to interfere in Hogwarts in order to fix various problems with the way the school it run. They're doing it more for personal gain and paranoia about Dumbledore. However, the fact of the matter is that if a real school was run like Hogwarts (for example, letting a teacher bring dangerous animals to class), it probably would have been shut down.