I've seen four main complaints leveled against the book.
1) It's boring, not much happens, etc.
I think it could have done with less of the teeny romance and more of the Voldemort stuff, yes. But I also think the slower pace suited it, as it was more a character study than a Magical Romp. I would have liked more of the Voldemort study, and some more about the parallels between Voldemort and Harry, but what there was, I enjoyed very much.
2) The characters don't act like themselves.
I can understand this one a little, but...looking back, I can also understand where the changes come from. In the case of most of the characters, they were simply growing up. Hormones--which had been bubbling rather strongly for at least two books prior to this one--kicked in. I think less time could have been spent on them and more on the more interesting bits of character, but eh. Harry himself was somewhat different, but that's because his growing up is more traumatic than most. He's continued to grow angrier--ever since PoA, in fact, when he found out more about his parents' fate--and there were even moments in this book where he was actually interesting. Of course, there is the matter of one more character, but I'll get to her. I think people know of whom I speak.
3) It was all about stupid romance!
This did get tiresome after a while. But I think it was inevitable. They are going on seventeen now. Rowling is handling the matter reasonably well. And there were some very touching moments even on the romance front. I adored Mrs. Weasley's conversation with Fleur towards the end--that actually had me choking up a little, that simple moment of hesitant, reluctant bonding--even though I could see it coming since the revelation of what had happened to Bill.
4) GINNY SUE.
...yeah. She really, really sucked. I wanted to punch her lights (and some of her teeth) out several times. Near the very end, when she was talking with Harry, she actually started to develop some proto-personality, and I kind of liked her. So long as a character has personality, I have no problem with them being Too Good To Be True. I mean, Dumbledore was essentially perfect, save for his relentless optimism about people, and he was a great character. If Rowling gives Ginny actual personality in the next book, as well as a key flaw or two, I'll forgive her for this one.
The other big thing I had a problem with was the ruining of Snape's character. What made him so genuinely interesting, so much one of Rowling's best characters, was that he was an asshole, that he was mean and even cruel, but not evil. Now he is. I'd like to hope against all hope that he's still not evil, that there's a more complex motive at work: that he killed Dumbledore knowing that he would have been killed anyway, and that Dumbledore knew this as well...but I'm not going to hold my breath.
The main thing I liked was that Voldemort is no longer a shapeless mass of raw evil but a surprisingly interesting character, a formidable villain and well worth exploring. I like characters. I suppose that's why I liked this book so much, despite its flaws.
In conclusion: a worthy lead-up to the conclusion, and I cross my fingers that Rowling can deliver again, and maybe even fix some of the mistakes she made in this one.
In additional conclusion: Heart Fleur, heart Luna, would heart Ginny if someone would chisel a few flaws into her, might even someday heart Harry if he keeps up this "developing personality" business.
And perhaps, just perhaps, I will attempt this sleep thing. Not yet; I'm not tired enough. But soon...sometime...
And I will not write crossover fanfic where a very grumpy John Constantine gets dragged into teaching Defense Against the Dark Arts.
Edit: Aaah! Now it all makes sense! Very spoilery look at the book, with theories on <vague>the locket and Snape</vague> that satisfy me greatly.
Bride of Edit: I commented in someone else's rant about Major Spoilery Things, and for ease of access I'm reposting it here.
Mmm. I mercifully have avoided most of HP fandom over the years and thus was never much exposed to fanon Snape. I never, ever saw him as romantic. That would be absurd. To me, he was always a petty, mean, nasty little man--but he was also a petty, mean, nasty little man who did basically the right thing when the chips were down. That's what made him one of Rowling's more three-dimensional characters.
As far as I'm concerned, if it turns out he was hardcore evil all along, the character just had two of those dimensions stripped away. I have no illusions about him as some sort of romantic figure, but what makes him an engaging character is the genuine conflict between his attitude, his pettiness and bitterness, and his sense of duty. I saw this and adored it from the very first book--this man who despised Harry, resented him fiercely, but felt that it was his duty to protect him. And probably resented him more for this. The spite was never an act. He may have played it up, but he truly felt it, all the same.
My main issue with evil!Snape is that it would go completely against JKR's grain. Since when has Harry been Right All Along? Since when has JKR shown us something blatant and obvious at the beginning of a book and not disproven it eventually?
One theory I could get behind (if I actually had more hard evidence for it; it came to me because it fits my view of Snape's character, but I'd have to comb back through the books for actual evidence, and I'm awfully lazy) is that Snape's value as a tool of the OotP is his ability to use that disgusting bitterness of his and not let it use him. Maybe that's why he could use the Killing Curse so easily--he was just so full of pent-up spite and anger that it's easy for him to tap into it at short notice. I'd be interested, too, in seeing what exactly makes him such a good Occlumens...if he uses his petty vindictiveness to guard his mind against intrusion as well. But of course, all that is speculation.
I'm a little bemused by your casual insistence that everyone who thinks there was more going on than Ha Ha Snape Is Evil is deep in denial, though. While I don't doubt that many people are (HP is a godawful pit of fanwank, after all), others simply find kinda-sorta-good-or-at-least-loyal!Snape more consistent with the series so far. As I mentioned earlier, it would simply be out of character for JKR herself to make things so plain. Especially in a book that's more of a character study than a Magical Romp.
Besides, the idea of Snape being Dashingly Heroic or even The Oppressed And Brooding Antihero With A Deeply Romantic Soul vomit-inducing. What has always made him interesting to me is that he is mean and spiteful and petty and stupidly, cruelly vindictive and not a bad guy.