I read Order of the Phoenix yesterday in one big gulp, from about 11:30 (went out to get it in the morning) to about 2:30. The rest of the day I spent socializing (we have a houseguest) and intermittently trying to digest.
What follows is my impressions after a night's sleep, but not after a re-read: a detailed one is up next. These are my impressions of the whole book, and therefore have huge, book-destroying SPOILERS.
Right. It's my immediate impression that this book has much less fat in the plot than Goblet of Fire. There are two distinct plot threads—the Ministry and Voldemort—that don't really come together, but I can accept that because we're much more into a continuing story right now, and the story isn't confined to Hogwarts anymore. The beginning of the book signals this quite clearly: Harry listening for the news, the hearing, and the time spent with the Order. We need to see how the wider wizarding world is affecting life everywhere, including at Hogwarts. And again, it's my current impression that most of the events we see have to do with one or the other of these plots, with little that's extraneous: Quidditch, Cho to some extent, and that's about it.
As two asides: ugh, fat = bad still there. (The pen was a perfect stroke of evil genius, but I'm not willing to forgive the appearance of the character for that.) And I'm not sure I find Fudge's capitulation at the end credible: I'll have to look at that closely on the re-read.
However, I'm presently quite mad with Harry about the Voldemort plot. Yes, I think Dumbledore is at fault for not sharing more information with him. However, Harry was quite clearly told by Snape that Voldemort could affect his thoughts and actions now, and so even without Dumbledore's information, Harry knew how vital his studies were. And yet he blew them off because he "was so intensely curious about what was hidden in that room full of dusty orbs that he was quite keen for the dreams to continue" (p. 682 US).
I think you can take the rant as read, here.
Sirius: I'm more sad for the loss of potential, both for himself and for those around him, than the loss of the actual character; I was never that attached to Sirius in canon. I do worry quite a bit though about the ongoing process of isolating Harry; beyond the psychological effects, I worry that he won't have people around him to smack him upside the head and tell him that he's wrong. Note that Dumbledore doesn't, even when he confesses that he didn't practice Occlumency. Perhaps the D.A. crowd, in particular the ones who participated in the Department of Mysteries raid, will pull together around Harry; but I expect that he's going to pull away just as hard, now that he knows this prophecy.
Also: Sirius is definitely dead. To my reading, it's clear that the veil is the literal manifestation of the boundary between life and death, on the other side of which is "the undiscovered country, from whose bourn / No traveler returns." He's not coming back. If he does, I will throw the book at the wall. Hard.
The D.A. crowd: I agree with some comments that I've read that the appearance of Ginny with a personality was quite abrupt, but I think it's long overdue so I don't mind so much. And I am just thrilled at Neville—especially the stabbing the wand into the eye—though I am annoyed that Harry didn't think to Finite the Tarantallegra. Luna, eh, well, we'll wait and see.
I was not particularly involved with the other new characters; again, we'll wait and see.
The prophecy: it's odd to think of Harry Potter as most recent in the tradition going back to Oedipus, but there you go. This should be an entry on the Evil Overlord list: I will not take action on any prophecy unless I'm sure I have the entire prophecy and have had it parsed by six lawyers, four philosophers, and two young children. I'm not sure what to make of the possibility that it could have referred to Neville, but I think I like it.
Very little actual romance; I doubt we're going to see any actual romance front-and-center during the books. Comments on the character development of the Trio are going to wait. Slytherins still uniformly presented as rotten, but I think that's going to have to change in the next books—though we're really starting to run out of time.
How does the Wizarding government work? Do they have an actual legislature, does the executive branch just pass things by decree? I note that the executive and judiciary seem to be mixed, at least from the evidence of Harry's hearing (I forget what we know about the justice system from past books, will have to look again.) Something very screwy is going on there in terms of separation of powers and safeguards.
Hmmm. I think that's all at present. Re-read begins: now.