Inevitably, a lot of details and subplots are going to be left out of any adaptation and given the length and complexity of the Harry Potter series, much more had to be left out than any devoted fan is going to like. The fifth book is the longest in the series and has a lot of subplots and threads of character development. Certain omissions are fairly easily forgiven - Quidditch, Ron and Hermione becoming prefects, studying for O.W.L.'s, career counseling - while others are rather disastrous. Harry Potter and the Order of the Phoenix is right in the middle of the series, so inconsistencies, whether of plot, character motivation, the basics of using magic, or design, render the whole series less coherent. And, most unfortunately, much of Rowling's marvelous humor and unpretentiousness gets tossed out in exchange for Drama with a capital D culminating in sappy bathos. I have a lot of complaints, but these are the worst, the most unforgivable problems in the fifth movie.
1. Voldemort wears a suit.
When Harry is boarding the train to return to Hogwarts, he sees Voldemort in a well-tailored Muggle suit. Granted, Harry is either imagining or hallucinating him for reasons that are unclear, but seeing Voldemort in a suit isn't scary at all. It's ridiculous. I'm immediately seeing Voldemort walk into a high-end men's shop and fussing about seams and the like, while the salesman tries not to stare at his flattened nose. If Voldemort were wearing his typical robes, this scene would at least have a shot at being scary, but as it is, it's dreadful.
2. There is no excuse for bad CGI with a budget that big.
We know that the Harry Potter movies had a good visual effects team because they did great work on Dobby and the thestrels, plus huge amounts of money were poured into the effects. And even so, the centaurs look absolutely terrible. Their faces are flat, like plastic masks. Why, oh why, did they not use people for the person-part and horses for the horse-part? It worked in the BBC Chronicles of Narnia and they were made in the late eighties and early nineties. The CGI is really bad if a guy superimposed on a horse in front of a green screen looks better.
3. Flying to Headquarters breaks the Statute of Secrecy.
At the beginning of the movie, Harry is rescued from the Dursleys' house and flies with members of the Order of the Phoenix to Headquarters at Grimmauld Place. He is facing a hearing for using magic while still underage and while in the company of a Muggle. So, naturally, he really shouldn't break the Statute of Secrecy again, right? Well, he and his escort fly so low that they almost crash into a boat on the Thames. And, if you look closely, there are Muggles on that boat. So, Harry, already in danger of expulsion and the permanent loss of his wand, along with various members of a secret, subversive society, go ahead and practically ask to be seen by Muggles while flying broomsticks. Special effects shots should never contradict the logic of the plot and this is an egregious contradiction.
4. Prophecies can't talk.
I almost understand why they decided to have the prophecy talk. The audience does need to hear the prophecy because it has a huge impact on the rest of the plot. The problem is all in the timing. Harry hears the prophecy before he faces Voldemort, so he already knows that "neither can live while the other survives." In the book, Dumbledore relates the prophecy to Harry in private after the events at the Ministry. Once Harry has heard the prophecy, he knows that he can't simply win and he can't rely on Dumbledore, but he needs to believe both of those things throughout the finale at the Ministry because he fights Voldemort with hope, both in Dumbledore and in his own ability to survive. And if Harry can hear it, why can't the surrounding Deatheaters (who would have accomplished their mission without lifting a wand) or all of Harry's friends? There are similar problems with the talking letter at the beginning of the film (only a Howler talks), which ironically is marked confidential.
5. Gary Oldman apparently neglected to read the books.
Sirius Black is one of the most important characters in the whole franchise. He's rebellious, bitter, reckless, and damaged. Gary Oldman's interpretation of Sirius is the complete opposite. He's understanding, resigned, and resembles most closely an over-eager therapist. He feeds Harry stuff like "You're a very good person who bad things have happened to," instead of encouraging him to form subversive school groups, as he does in the book. He has no inner demons, which is bizarre given that he was rejected by his own family, lost his dearest friend, was framed for his murder, spent more than a decade in Azkaban, and is forced to live isolated in a house redolent of his worst childhood memories. What the hell, Gary Oldman? I know you can play traumatized and bitter loneliness, I just know it - you did it in the third movie. And he also wears a nauseating velvet jacket, but at least that's not his fault.
6. Plot holes, plot holes, everywhere.
I could find several dozen plot holes, but I'll confine myself to two of the worst, neither of which would have required a lot of time or expense. The scenes of cleaning Grimmauld Place are cut out, which means we never see a certain locket, which in book seven is revealed to be a Horcrux. In the films, Harry has never seen the locket, and neither has Mundungus Fletcher who later steals it (Mundungus isn't even in the fifth movie). The appearance of a Horcrux is kind of important. Far worse is the exclusion of the two-way mirror, which Sirius gives to Harry as a gift. Part of Harry's guilt over Sirius's death is created by his forgetting the mirror, which would have let him know that Sirius wasn't in the Department of Mysteries at all, but even more importantly, it's by means of this mirror, which Harry has kept for purely sentimental reasons, that he and his friends escape from the Malfoys' estate in the seventh movie. I realize that not everything can make the cut, but both of these examples are important to the series as a whole.
Two hours and one minute into the movie is a clip of less than five seconds that is so extraordinarily awful that it warrants it's very own paragraph. It is a clip of Ralph Fiennes doing a noise that sounds like "Pah" and looking utterly silly and decidedly not scary, sticking out his tongue like Gene Simmons. It occurs right at the climax of the film, when Harry is fighting to prevent Voldemort from possessing him. If it weren't for that stupid "Pah," the montage of memories and Voldemort being scary would be really good. With the "Pah," I spend the last fifteen minutes of the movie pondering the absurdity of the "Pah" and fervently wishing that it weren't there.
What they got right: My favorite scene in the movie is simple and short. Harry is telling Ron and Hermione about his first kiss. All three actors acquit themselves beautifully and it's a scene that is funny and warm and reminds us of what the series is really about: the power of friendship.