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.
Ass Pull: The revelation that the reason Harry's Invisibility Cloak never tears or falls into disarray is because it is one of the eponymous Deathly Hallows is a clever inversion of this, much like the rest of Chekhov's Armory in the books. Harry takes the power of his Cloak for granted, simply assuming that all Invisibility Cloaks are just that awesome, but attentive readers will notice Mad-Eye Moody talking about having to borrow a fellow Order member's Invisibility Cloak because his was starting to wear off. The average reader would wave it off as a continuity error, but it it becomes an important plot device in the final book.
On the other hand, Dumbledore's claim that the Cloak's true magic is that it can hide multiple people is a more traditional Ass Pull. So if anyone ever manufactures an oversize Invisibility Cloak, does it automatically get promoted to Hallowdom?)
Not its true magic, but its true worth as a Hallow. The Wand and the Stone were for more selfish uses in comparison.
But couldn't the Elder Wand just as easily be used to, say, cast a Shield Charm over one's friends? Or the Cloak used to steal from someone? There's really nothing inherently more moral about any of them, it's just that the particular people who got them used them in particular ways.
The fact that Fiendfyre could destroy horcruxes was not mentioned until after it burst out and attacked everybody. Hermione handwaves it by saying it was too dangerous to attempt, since its flames hunt down its victims.
Ron enters the Chamber of Secrets. By learning to speak Parseltongue. Only not really because he was only imitating the sound of it. And it let him in anyway.
The Deluminator comes off a bit like this, as it's one previous appearence in the first book gave no indication that it could be used the way it is in this one.
Award Snub: Considering that Deathly Hallows Part 2 was not only the highest grossing movie of the year, but the highest grossing installment of the highest grossing franchise, and one of the highest grossing films of all time, in addition to being one of the most critically acclaimed films of the year, would have at least garnered a few token nominations at the Academy Awards, especially since the Academy had made the decision several years earlier to expand the Best Picture field to ten films. But no, just three nominations in the technical department which it lost in all of them (the worst being losing Best Makeup to The Iron Lady! It is also suspicious that before the nomination process began, the Academy again ammended it's rules that up to 10 films could be nominated for Best Picture, if they gathered a certain percentage of votes. Coincidentally, only nine films were nominated, which prompted much outrage from both fans and film critics.
If you were wondering, these were Warner Bros. submissions to the Academy: Best Picture, Best Director - David Yates, Best Adapted Screenplay - Steven Kloves, Best Actor - Daniel Radcliffe and Rupert Grint, Best Actress - Emma Watson, Best Supporting Actor - Ralphe Fiennes and Alan Rickman, Best Supporting Actress - Helena Bonham Carter and Maggie Smith.
Some will certainly view the dance between Harry and Hermione after Ron's departure in the first film as such. Although it serves to illustrate what is never explicitly stated in the film: that Harry and Hermione are like siblings and will never be a romantic couple. Trying to show that with a scene that many (even non-shippers) saw as very romantic, was probably not the best way. Aside from romantic information, the scene serves to show how much shit has hit the fan and that no amount of having fun will lift their spirits and build Harry's confidence to take on his inevitable task.
Probably the greatest of all of them being when Neville, the series' perpetual Butt Monkeypulls Gryffindor's sword from the Sorting Hat and slices off Nagini's head. Even more awesome because this is what makes Harry's final victory over Voldemort possible, by eliminating the last Horcrux and making him mortal again. What makes it even more awesome is that he is ON FIRE while doing it, having been set on fire for refusing to kneel before Voldemort.
Snape is now the headmaster and announces that Harry's been sighted on campus, but instructs a clearly relieved crowd of students to report any information they have to him or be punished. Oh guess what!? HARRY'S RIGHT THERE HIDING IN THE CROWD, AND IMMEDIATELY CALLS SNAPE OUT, says he is insulting the man who used to stand there, and calls his back-up cavalry to aide him! Everyone is overjoyed.
Die for Our Ship: After announcing that Harry and Hermione would not become a couple, J.K. Rowling was actually getting threats of boycotts (as well as death threats) from rabid Harry×Hermione shippers. She apparently brushed them off with this book as a big Take That.
Draco in Leather Pants: Well, it had to happen eventually. After the defeat of Voldemort, Draco reforms and, in the epilogue, ends up being just another parent sending his wizard offspring to Hogwarts.
Fan Dumb: For some insanely idiotic reason, Tom Felton was nominated for Best Villain at the MTV Movie Awards, for Part 1. Even though anyone who watched the film (or, even better, read the book) know that it only proves that Draco is not evil as we had thought. Considering Voldemort's minimal screentime in the first film, it would've made more sense to nominate Helena Bonham Carter instead, especially since she graphically tortures Hermione and kills Dobby.
Fanon Discontinuity: Some Harry/Hermione shippers think that JKR wrote the "nineteen years later" epilogue primarily or solely to spite them by making "Harry and Hermione get together after Hogwarts" stories uncanonical. A number of fans reject the canonicity of the epilogue to get around this, leading to the phrase "Epilogue? What Epilogue?". It doesn't apply only to shippers. Quite a few people disliked the Epilogue for its reputation of "Tastes Like Diabetes."
"Funny Aneurysm" Moment: During the wedding, Fred comments that when he gets married, everything will be mellow and he'll put Molly in a Body-Bind Curse until it's all over. While this sort of speech is typical of Fred and George, it becomes quite tragic after you've finished the book.
Genius Bonus: Hermione was named after the character from Shakespeare's The Winter's Tale, which begins with a man who believes that his wife (named Hermione) is cheating on him with his best friend. There's also the fact that Hermione is the female form of Hermes, being the god of cunning in many Hellenistic cults.
Harsher in Hindsight: Remember Molly's boggart from Order of the Phoenix? It comes true with Fred's death at the end of the book.
The "NYEAH!!" scream of Voldemort that become famous thanks to the Part II trailer was actually heard once in Part 1, right after Harry's wand overpowers him in the aerial battle.
Jerkass Woobie: Aunt Petunia. Yes, there's no excuse for the way she treated Harry, but in the book we see that her "you're a freak" attitude grew out of insane jealousy. Imagine being a little kid, seeing all the goings-on at Platform 9-3/4, and knowing you'll never, ever get to be part of that world.
Memetic Badass: Neville Longbottom will reject your bribes and kill your pet. Neville Longbottom uses Basilisk venom as soy sauce. Neville Longbottom would have done it in four books. Neville... you get the idea. Chuck Norris's patronus is Neville Longbottom. Thereby making Neville the Wizarding World equivalent of Chuck Norris.
"NOT MY DAUGHTER, YOU BITCH!!!!", which made Molly Weasley ascend to Badass status.
On a scale of one to Lord Voldemort, how awkward are your hugs?
Voldemort's scream from the trailers: NYYYYYYYYYYYYYYYYEEEEEEEAAAAAAH!!!!!
Magnificent Bastard: Once you've come this far in the series, it's settled. This title doesn't belong to Voldemort, it goes to none other than Albus Dumbledore himself. Directly and indirectly, intentionally and otherwise, this one man, has been responsible for everything, everything, that has driven the history of the Potterverse from his time and even beyond, fitting every characteristic trope almost all the time.
Moral Event Horizon: Throughout the books, Malfoy's sidekicks Goyle and Crabbe were mostly ineffectual rivals to our trio, but towards the end, Crabbe actually goes Complete Monster and almost kills the three, even when Draco is dead set against it.
Narm: After he (seemingly) kills Harry during the climax of the second film, Voldemort approaches the school's defenders and offers them the chance to join the ranks of the Death Eaters. Draco, after hesitating, steps forward to join his parents, and is hugged by Voldemort on the way. Needless to say several theater houses were in stitches during that bit. This was probably intentional, considering Draco reacts like he's being hugged by, well, Voldemort (and is only doing it so he can leave with his parents) and Voldemort is pointing the Elder Wand at the other students the entire time
After being attacked by Voldemort, Neville wakes up and sees the sword with the derpiest possible expression.
In the same scene, Voldy exclaims HARRY POTTER IS DEAD! and uses the Death Eaters as a laugh track. Also, any time Voldy uses AVADA KEDAVRA!
The worst offender is when Voldemort casts it in the Forbidden Forest. "UHHVUDUH KEDRAVRUH!!!"
Neville's speech in the film sounds like something out of an after school special, when the mood of the scene demands something more along the lines of Leonidas' "Persians! Come and Get Them" or Theoden's "We Shall Have Peace" speech from the Return of the King.
Bellatrix and Voldemort dissolving into flakes for no apparent reason in the climax.
Snape's death. Nagini coming out of Bathilda Bagshot's neck. Harry even decided to spare Hermione the details of Nagini's arrival.
The guardian spell of the Slytherin locket.
What Ron sees when he destroys the locket. Made even worse when you realize that the locket was probably telling him those exact same things, over and over, in the time before he ran away.
In the movie, during Harry's tussle with Voldemort on the Hogwarts towers. The movie has Harry ride along with Voldermort as he tries to kill him around and through the building! And at one moment, the camera zooms in to show that the two characters' faces have merged! shudder
The scene where Harry uses the Cruciatus Curse on Amycus Carrow. Thankfully, this scene was omitted in the film.
J.K. Rowling chose to sink the Harry×Hermione shipthrice, the third time of which was in this book. First, in an interview about the possible pairing, Rowling said she didn't think they were suited for each other. Secondly in Half-Blood Prince, and then the internet exploded, with Harry/Hermione shipperssending out death threats, burning books, and swearing to abandon the series if she didn't change it. To drive the nail in the coffin, this book has Harry giving the "Like Brother and Sister" speech to Ron and Hermione, right after a scene in which Harry/Hermione appears as an evil illusion made by the locket-Horcrux that seems vaguely perverse and is intended entirely to enrage Ron into attacking Harry by using frequent Harmonian arguments, at that.
She also said Neville×Luna wasn't canon. On the other hand, the movie has Neville tell Harry he has to tell Luna how he feels. And There Was Much Rejoicing for the Neville×Luna shippers. Which was sunk again by Matthew Lewis (who plays Neville in the films), when he said the Neville×Luna relationship was ultimately only a "summer fling", and that they did eventually move on to their respective canon pairings.
Ship-to-Ship Combat: Or rather Ship-to-Ship Last Stands, as this book effectively crushed the hopes of numerous shippers, but especially Harry×Hermione shippers. However, true to the nature of shipping, many prefer their ship than what was written in the books. And why should they stop liking it?
Tear Jerker: In the film, we see Hermione Obliviating her parents' memories of her, preceded by Hermione's wavering "Coming, Mum" as she steels herself to do it.
The Snape Pensive Sequence. Seeing Snape wail while cradling Lily's body will rip those tears right out of our eyes, mark this troper's words.
Not to mention baby Harry crying behind them.
Snape casting his Patronus.
Basically the entire movie.
Lupin and Tonks reaching for each other's hands. This is the last time that we see them alive, and they never touch hands, which is made even more heartbreaking by the fact that when we see their bodies, Tonks' arm is still reaching out, but their hands still aren't touching.
The use of Leaving Hogwarts in the epilogue.
The Untwist: Wormtail's life debt to Harry isn't repaid out of gratitude or a Heel Face Turn, but instead he's simply magically prevented from harming Harry at a critical moment. His "mercy" is completely accidental, and then triggers a You Have Failed Me failsafe...
Not necessarily, it's a question of how you read the scene. When Harry reminds Wormtail that he owes him, there is a short pause before Wormtail strangles himself. He is stated to have fear and surprise painted in his face, and him having "a moment of weakness" is mentioned. It is possible that his silver hand strangles him just as if Harry's words order it to, but it is also possible that Wormtail actually hesitates because he remembers that Harry saved his life, and before he can decide what to do, his so-called "moment of weakness" makes the hand strangle him (it's wouldn't be completely unthinkable for the hand to react to those sorts of feelings since it was created by Voldemort himself who doesn't exactly likes weakness).
Trapped By Mountain Lions: The main storyline falls victim to this, which is the only point of view given after the first chapter. Obviously, Harry, Ron, and Hermione hunting down the Horcruxes is a big deal, but with its difficult pacing and long stretches of Dumbledore back story only tangentially necessary to the plot, it sort of falls short compared to Voldemort having taken over the Ministry of Magic and NevillerunningLa Résistanceinside the school, which the Golden Trio (and thus the readers) only hear about secondhand.
Unfortunate Implications: Lupin saying that werewolves don't usually mate, feeling unworthy of his family due to discrimination and overall minority angst makes you wonder whether children of minorities that are oppressed in real life (especially those isolated from others of their kind) will get the impression that their minority status leads to being miserable.
For what it's worth, Word Of God says that lycanthropy is supposed to be a metaphor for AIDS. Fear of having a werewolf kid makes a bit more sense from that angle (though, since it doesn't actually seem to transfer to sexual partners or offspring, one could argue that the metaphor is a bit off).
What an Idiot: Yes, to safeguard the Horcrux, let's wear it at all times. It's not like we had proof that said Horcruxes can posses or damage the bearer or had a handy bag to put it in. And the feelings of paranoia and anger, plus the deterioration of friendship... totally normal. In the book, the trio knew that the Horcrux affected them but still carried it everywhere. It's a bit stupid, especially when Hermione has a wonderful moleskin poach that can keep many things inside and shrinks down when needed to prevent it from being stolen...
Bags get dropped, especially when running for your life. Only way to be sure you don't drop the really really important thing that everybody's fate hinges upon? Put it around your neck. Unlike the One Ring, this pendant isn't inclined to make escape attempts.
Except when Harry ends up underwater, the pendant tries to drown him by choking him to death, while simultaneously getting heavy enough to keep him under. In retrospect, wearing a cursed object around your neck isn't a very good idea either.
Admittedly there is a good reason to wear it. If you don't have somewhere to keep it when you're not wearing it. During night and sleep, they could have kept it on a table in the tent, that way they could glance at it, and only wear it when all of them were walking about. It would ahve saved a lot of trouble for them.
Snape, particularly as a kid and teenager where he keeps messing things up with Lily despite how hard he tries. However, certainly being unable to protect the woman you love and then protecting her child who was sired by and looks exactly like the man you hate has got to be the rawer end of the deal.
Pius Thicknesse is one of the biggest woobies of the series. His life's goal was to be Minister of Magic — but he took it for granted that he would be himself if/when he made it. Not to mention the sheer evil he was made to participate in.