Go back now if you don't want to be spoiled.
No pun intended.
Also, I have at least one theory related to Deathly Hallows that stems directly from this movie, and things I've read about it. You are thus doubly forewarned.
Before going too in depth, let me just say something: unscientifically comparing my copies of Order of the Phoenix and The Lord of the Rings reveals that the two are more or less the same size. LotR was made into three 3-hour movies and still took flack for leaving out Tom Bombadil and the Scouring of the Shire, among other things. Order of the Phoenix was made into one 2 3/4 hour long movie. Stuff had to be sacrificed.
All in all, I think they did a reasonably good job deciding what to keep and what to sacrifice.
For the record, the major plot points that I can recall and pinpoint as being sacrificed are, in no particular order:
- Weasley is Our King: Quidditch
- Ron and Hermione being Prefects (note that they may have obliquely referred to this for just a tiny moment)
- Percy (also obliquely referred to by having him show up twice with Crouch)
- Most of Kreacher's storyline (see below)
- Luna's role in the Quibbler is much reduced here
- Meeting Neville's parents (who are referred to but not shown)
- Firenze teaching Divination
- The Weasleys' Revolt (though what it shown is awesome)
- Weasleys' Wizard Wheezes
- Harry's angst at the beginning
- The Torture of Professor Trelawney
- Sirius chafing at his enforced confinement at Grimmauld Place
- Harry and Cho
- Mrs. Figg being a squib and looking out for Harry all along
Things that seem to me to be different than in the book:
- I don't recall Cho being the one to betray Dumbledore's Army, even under Veritaserum
- Likewise, I don't recall Filch camping out in front of the Room of Requirement waiting for the DA to leave
Things that rocked:
- Professor McGonagall: "Oh, there are quite a few things I would like to say to you, Sybil!"
- The Weasleys' Revolt. Just 'cause.
- Fred and George comforting the boy that was in detention.
- Believe it or not, the Occlumency lessons. I liked the visuals.
- The thestrals. How cool are they? I would just liked to have seen the others' faces after flying to London on animals they couldn't see.
- Ginny destroying the Department of Mysteries with one spell. Heh.
- Hermione flattening Ron, and the twins betting on it.
- The poster of Fudge getting shredded during Voldemort and Dumbledore's battle. I just liked the symbolism.
- Harry's angst. I defended it when the book came out, and I defend it now. The kid had been through a lot and was entirely entitled to be cranky.
- The acting, especially from the Trio. While still not Oscar-quality actors, they improve with each film. In this one, Emma Watson seemed to show the most improvement.
Things that were okay:
- Sidestepping showing Neville's parents by having Sirius and Neville both speak about it.
- Luna. The actress was good, but not great.
- The visual effects. Some were awesome, some not so much. I particularly liked the Veil; I particularly disliked the obvious replacement of Emma Watson with a CGI'd image when Grawp picked her up.
- Mr. and Mrs. Weasley. "Daddy's home?" Mwuah? But on the other hand, the enduring affection they have for Harry -- off-screen, you can hear Mrs. Weasley saying something like "he's like one of our own" -- is lovely.
- Dumbledore. I just can't buy the new Dumbledore. He's too...forceful. At least he didn't grab any kids this time.
- The Snape-James incident...the point of that was what it taught Harry, and that got left out entirely.
Things I didn't like:
- Bellatrix. I admit that this is the first Harry Potter movie I've seen without re-reading the book, but I don't recall her being so over-the-top crazy. I recall her more as being...well...quietly, scarily crazy.
- Shunting McGonagall off to the side...I really didn't like that they didn't show her part in the Wesleys' Rebellion. But given that I recall production stills show her sitting, exhausted, in a wheelchair, perhaps she couldn't handle more than her role was.
Overall, I thought this was a very successful film, though it takes the film franchise further from the story in the books, particularly in regards to secondary characters. Ron, especially, has suffered from the parts that have been dropped. But, like Lord of the Rings, these films are creating a universe separate from that created in the book.
They're related, certainly, and they tell the same essential story, but the narrative is focused on Harry and Voldemort almost exclusively. While the books are written from Harry's point of view, their focus is broader.
But that brings me to a point of speculation. Sci-Fi Wire had an article a while back that said that while J.K. Rowling isn't greatly involved in the movies, she does offer suggestions and information, including a sketch of the family tree in Grimmauld Place, and suggesting that they not remove a character from Order of the Phoenix because they would have trouble when they made the film version of Deathly Hallows.
My suspicion, based on how little both are in the film, is that the character is either Kreacher or Percy.
Kreacher is suspect, of course, because of the Black family's history as Dark Wizards, and because he has already betrayed the Order of the Phoenix once. I can envision many roles for Kreacher to take in Deathly Hallows; of course, I won't know for a week. But the fact that house elves are powerfully magical makes him a suspect.
The other is Percy. Percy is shown very little in the film, and the drama with his family is largely ignored. Unfortunately, I don't see his story ending well: he seems solely focused on procuring power. He's also seemed to be very suggestible and not very savvy. When Percy is shown in the film, he's acting directly against Harry, and is always shown with Fudge and members of the Ministry.
It could be that Percy plays a prominent role in the last book -- and therefore film -- and that J.K. Rowling felt that it was important that his shift in attitude at least be briefly addressed.
Ironically, despite the almost ritual reports now of people complaining that Order of the Phoenix is just getting too dark (there was even a Yahoo! headline a few days ago to that effect), I found it less intense and certainly less scary than Goblet of Fire. The scene in the graveyard in the latter was so well-done, and the atmosphere itself so frightening, that it outdoes the Department of Mysteries in my mind.
Harry Potter and the Order of the Phoenix was a good book that moved the plot of the larger story along, but it lacked the introspection of Prisoner of Azkaban or long-awaited answers of Half-Blood Prince. It's a good, but not great, book.
I'd say the same of the film. It's a film that had to happen to progress the story into the coming climax. It's good, but not great.
I'd say, ultimately, give it a 6 or a 7 out of 10. I may change my mind after seeing the Department of Mysteries stuff in 3D (the IMAX theaters were sold out).