I read the book this weekend. I’d have to give it a B-, simply because Rowling is in terrible need of an editor.
The first 250 pages are incredibly tedious, which is unfortunate because, as Amp said, it explores entirely different areas of Rowling’s world than we’ve seen before.
I was most disappointed with her depiction of Harry. Sure, she’s got the whole “teen angst” thing down pretty good, but that doesn’t mean he’s supposed to run around like a complete moron. He makes more stupid mistakes than is plausible. Harry from “Sorceror’s Stone” wouldn’t have made this many mistakes. I think an editor really would have improved that quite a bit. Around page 300 he finally gets a clue and acts reasonably, IMO.
I’d rank “Order of the Phoenix” #3 of the HP series, with “Prisoner of Azkaban” at #1 and “Goblet of Fire” at #2.
As for the death, I had it spoiled for me, so I can’t quite tell you about that. Having known, though, I could see Rowling had laid a lot of traps for the reader, leading to many different possible outcomes to that particular answer. That’s all I reveal, in case I give too much.
The new characters are absolutely wonderfully written - Rowling’s strong suit. She beefs up Neville Longbottom and Ginny Weasley to my delight (I love those characters, and was really looking forward to them in particular). I’d really like to see a short-story collection from her between this book and the next, giving some sidestories/backgrounds on the peripheral characters. Hermione before Hogwarts, Neville’s parents’ death, James and Lily Potter’s wedding, etc. would be wonderful.
All in all, I think this book was designed to get the story some serious momentum. It’s trucking right along - I don’t expect a lot of twists and turns in the next two books, but I do see a lot of action in the future.