On horcruxen, to begin with:
The locket: There's a theory that the Slytherin locket which is one of the horcruxes that Voldemort made, was stolen by Regulus (whenever I see that name I think of the cruise missile) Black, and found in Order of the Phoenix at 12 Grimmauld. Now, I think this is possible, but if so, only as part of A Better Idea. Because, the Slytherin locket ought to be marked as such, and I think that it would be sufficiently notable an artifact that someone would have recognized it, and given it to Snape if nothing else. In short, I don't think it would have been left lying around if it were the real, ancient thing.
Harry as a horcrux - Ok, I see where people are thinking. kate_nepveu has a nice post that discusses the prerequisites. Personally, I'm not convinced, although I think it is plausible. One of the objections to this is that if Harry or his scar were a horcrux, Voldemort wouldn't be trying to kill him. Now, it seems to me that given all the fear Voldemort has for Harry, it would be real easy to view this as a form of chemotherapy, and who cares if it kills 1/7th of his soul? He's got plenty more stashed elsewhere, and once he kills Harry, he's sure he's home free.
Am I the only one who missed Quidditch being treated at all seriously? I mean, for something that was a core theme for Harry a few books ago, he now becomes the Captain and suddenly it doesn't matter. I suppose it hit page count limits.
The pedagogy at Hogwarts still appears to be atrocious. I'm amazed that they learn anything, but then again when you look at it, which the exception of Hermione there isn't much indication they do anyway. The speculation that Harry will become the Defense Against the Dark Arts teacher very soon, like upon graduation, is well, rather disturbing. Harry has much experience, but little skill, at DADA. He appears to be an effective teacher, according to the testimonies of the Weasley Bros.(1) and experience with Dumbledore's Army. BTW, there is one private army in Britain in the Muggle world, I wonder if DA intends to be the equivalent.
JKR seems decidedly opposed to the anti-Voldemort measures the Ministry of Magic(2) has distributed. Given how they're depicted, I'd tend to agree. What is her proposal for what they should do, besides prepare Dumbledore and Harry a parade and genuflection every day?
(1) I just had this image of Yacko, Wacko, Ron and Ginny doing an Animagius Maniacs intro and then saying "We're the Weasley Brothers! And the Weasley Sister!"
(2) Minister to WHO? Unless they get along real well with Elizabeth II R.
To mention a non-spoiler HP topic, I'll wander into my thesis. In Afghanistan, there used to be (I understand its abated somewhat these days) two sort of societies, who accepted members from all the clans, tribes, etc. Their names worked out to "The Blacks" and "The Whites", and it seems pretty arbitrary who ended up a member of which. Their only purpose was to give brothers, uncles and other people too closely related otherwise an opportunity to be enemies over something. This from the culture which has the same word for "rival" and "father's brother's son" (cousin). Now, it seems to me that while the House differences aren't quite so extreme, there's still a lot of hostility between students in the different houses while at school, and the school actively promotes this. Why don't these rivalries carry over into Wizarding life in general? Why isn't there a store that won't hire or serve Gryffindors, or vice versa? After all, as far as we can tell everybody of importance in wizarding Britain, if not everybody, goes to Hogwarts. (Hmm. Has anybody tried to estimate the total size of the population if that's the case?)