There were already traces of adaptation troubles in the first two movies, but at least in SS and CoS, the director actually cared about staying true to the story. Alfonso Cuaron, the director for PoA, clearly cared more about being *~artsy~* than about an actual adaptation, and it detracted from telling the actual story.
So, let me get this straight: we can have an extended Knight Bus sequence with an annoying shrunken head, we can have a frog choir singing the Three Witches' chant from Macbeth (??? okay), we can have some weird sequence in the Gryffindor dorms with special effects candy, we can have more shrunken heads in Hogsmeade, we can have this ridiculously over-the-top scene at the Whomping Willow with Hermione squealing and getting thrown around (and we gotta make sure she lands on Harry, just so everyone knows that the movies prioritize teasing this bland relationship that was never hinted at in the books), and we can have shot after shot after shot of the Whomping Willow doing things to signify the changing of the seasons (which was...not a significant motif in the books? don't really see why it's necessary here). We can waste endless amounts of time on junk like that, but actually explain to us who Moony, Wormtail, Padfoot, and Prongs are and coherently explain the Marauders' backstory in the Shrieking Shack? Nooooo. People who only watched the movies have to take it on trust that there's an actual story in there, because the movies sure weren't going to explain it.
There's also the more important matter of gutting the characters, who as far as I'm concerned are the most important part of the Potter story. Hermione is the worst offender, and frankly, it all starts before we even get to the content of the movie itself, with the fact that they couldn't be bothered to so much as give Emma Watson a wig. This is the movie that started the "bushy hair = princess curls" trend, and every time I look at movie-3-onward!Hermione, I get the overwhelming urge to vomit.
Here's the thing about Hermione, guys—she does not prioritize her looks. She has bushy hair and the one time she bothers to deal with it is for a special occasion. It's wild and frizzy and unmanageable, and is the subject of mockery by her peers. She doesn't wear particularly fashionable clothes, and she's not the kind of person who would whine about what her hair looks like from the back. That's what makes her so relatable—she doesn't have to be pretty; her worth comes from the compassionate, clever, and courageous person that she is. But accentuating future model Emma Watson with fashionable Muggle clothes and pretty hair and generally making her look so "made up" was the filmmakers' first mistake, because it's clear that they didn't understand Hermione. They understood marketable female protagonists. So we haven't even gotten to the meat of the movie before Hermione's character has been savaged.
Now all of that said, a lot of people (yes, myself included) hate how Hermione looks because it subverts one of the greatest things about the character. But even if Emma had been given a wig and not been given more than the bare minimum of stage makeup, Hermione's portrayal in the films would still have been an insult to the original character, and it starts in full force with this one because Hermione is way too perfect in this. And really, what it leads to is the other characters getting dragged down with her, because when the story gets warped to suit Hermione's purposes, the characters and their relationships get butchered.
In the book, Hermione gets into fights with both Harry and Ron—the first time because she tattles to McGonagall in a panic because Harry's been sent a broomstick anonymously and she thinks it's from someone who wants to kill him, the second time because it looks like her cat Crookshanks ate Ron's rat Scabbers and she refuses to apologize even when all the evidence points toward this being the case. These reveal important things about her character: that she'll go behind someone's back or over their head if she thinks it's all for the best (she never expresses that particular suspicion to either of the boys before running to McGonagall) and that she has a very difficult time admitting when she's wrong to the point of alienating her friends.
They also reveal something about her relationship with Harry: that she isn't just Harry's bestest-y bestest best friend in all of bestie-dom; he disagrees with her and gets mad at her and refuses to speak to her and she doesn't try to apologize in order to bridge the gap with him, either. He sticks by Ron in the fight over Crookshanks and Scabbers because he feels that Ron is in the right, and he still has fun hanging out with Ron (while in the next book, he expresses agitation with constantly hanging around with Hermione when he's feuding with Ron because hanging out with Hermione is much more studious and boring).
The movies didn't really seem to care very much about this. Look, I understand that things simply got cut for time, and since Quidditch wasn't really a big deal from this movie forward, I can forgive the Firebolt thing being squished into the end. What I can't forgive is the way the Crookshanks-Scabbers argument is framed. Throughout the book, Crookshanks repeatedly attacks Scabbers, so Ron has every reason to believe that Crookshanks finally caught him and ate him, and all the evidence leads even Harry to think so. But in the movie, Hermione is presented as being in the right when she blows Ron's concerns off with a "Ronald has lost his rat", and she's then immediately vindicated in the next scene when they find Scabbers alive and well in Hagrid's hut (and then the film has Ron act like a jerk about it to her for good measure, which again did not happen).
In other words, the movie took Hermione's failure to keep her pet from attacking and killing her friend's pet and framed Hermione as the tragic little victim. "But," people will say, "they couldn't have dragged the argument out that long! They had time constraints!" Sure didn't stop them from all the time-wasting nonsense above. And by cutting out the bulk of that argument, they also cut out the bigger picture of what that argument represents: that Hermione is the kind of person who refuses to admit that she's wrong, and she alienates her friends because of it. And it also cuts something important about Ron, too: he's furious with Hermione over the whole thing, largely because (as he says himself) that she'll never admit she's wrong and she's not even acting like she's sorry, but when Hermione apologizes, he immediately forgives her and even starts trying to mitigate the issue by talking about how old Scabbers was and maybe he'll get an owl next time. All he wanted was an apology he was entitled to, and once he got it, he was satisfied. In the movies, though, he's much more unflatteringly petty.
Connect this to the most infamous scene, at least to haters of this film. In the book's version of the Shrieking Shack event, when the Trio wind up in there with Sirius Black, Ron—whose leg has been broken and who's facing a man whom he believes is a mass murderer—declares that "if you want to kill Harry, you'll have to kill us, too!", and he's ready and willing to make that sacrifice without hesitation (an act that Sirius directly compares to the kinds of things he and his friends were willing to do for each other). It's one of his greatest moments and demonstrates his perseverance, loyalty, and self-sacrifice.
In the movies, Hermione says this line, while Ron just sits there being scared. That's not a change you make because you're pressed for time, guys. It's literally the exact same line Ron says in the book. It would have taken, what, maybe a couple of seconds more for Ron to lunge forward as best he could, put himself between Harry and Black, and deliver that line? No, it's given to Hermione.
So to put it in perspective, Hermione's greatest failing in the book—stubborn refusal to admit that she could be wrong and rightly angering her friend as a result—is directly reassigned to Ron, and Ron's greatest moment in the book is directly reassigned to Hermione.
Lastly, to fully substantiate just how little the filmmakers cared about Ron, I point to the classroom scene where Snape puts down Hermione (although Snape's portrayal in the films is a whole other can of worms) for being an "insufferable know-it-all". In the book, Ron immediately fires up at once and snarls at Snape that "You asked the question and she knows the answer! Why ask if you don't want to be told?" But in the movie, it's "He's right, you know".
...Really? You want to devote the rest of that scene to Draco Malfoy making fun of Harry instead of showing Ron's fierce loyalty to his friend and instead running him down as a character? Are you serious?
There's a reason for this, and it's a simple one. Steve Kloves, the screenwriter, has admitted that Hermione is his favorite character. He claims it's as a result of her social awkwardness and bossy behavior as much as it is her intelligence and strength. But how can it be, when he refuses to actually write her that way? How can she be socially awkward when she's never seen doing things like trying to detract from a classmate's grief over her dead pet in order to prove a point, or refusing to admit that her cat could have done a thing that a lot of cats do and killed and eaten her best friend's pet?
Why does Hermione do things like be the only one to reach out to an angry and upset Harry in Hogsmeade when he finds out part of the secret about Black? Why is she the one clearly closer to Harry when everyone's crowded at his bedside after the Quidditch match? Why doesn't Harry have any of these moments with Ron, but plenty with Hermione? Why is Ron's character so underwhelming compared to Hermione's? Because she's Kloves's special little pet, so she gets all the personality and all the awesome moments with none of the flaws or failings to have earned them. She gets to have the special friendship with Harry while Ron plays third wheel, even though Ron is canonically the character with whom Harry enjoys spending more one-on-one time and who understands him better. It's all about making sure that we, the audience, know that Ron doesn't matter and Hermione's the real heroine and gosh, why isn't she the main protagonist, and how could she have ended up with that yucky old ginger, anyway?