The pacing was much better this time than in any of the previous books. I didn't feel at all that action was stretched to fill a schoolyear, which has bothered me about every one of the previous books. I welcomed that Harry started confiding in people finally--Mr. Weasley and Prof. McGonagall about his suspicions about Draco, and Dumbledore about a lot of stuff, but that made his not asking Dumbledore about the half-blood prince the worse.
Why would Slughorn have needed (or merely appreciated) Dumbledore's help cleaning up his house? And Harry asked why Merope couldn't have magicked herself food and shelter--if that were possible, without money, wouldn't the Weasleys not wear hand-me-down robes? These are the type of inconsistencies about Rowling's world (in addition to waxing Procrustean with the timeline) that makes me itch.
My goodness, JKR has been reading her fantasy books, hasn't she? Susan Cooper's Over Sea, Under Stone, both setting and cup; the entrance to the Mines of Moria in Fellowship of the Ring; Arthur's white tomb; His Dark Materials. And the Chosen One, jeebus, in addition to Harry's being now without parental figures just like Buffy.
Plot points and questions of a spoilery nature: Was Dumbledore a Parseltongue? He understood the Gaunts' casual everyday use of it. Is it realistic that Draco would have bragged about joining Voldemort even to his pack? Being in Slytherin doesn't require being evil.
"Neither can live while the other survives," but they've both been alive for two years now. Unless the prophecy is off, and Dumbledore did say not to put too much stock into it, but if it didn't deserve stock, why'd he hire Trelawney?
Why, as Potions teacher for five years plus however long before Harry showed up, wouldn't Snape have taught students the better ways to do Potions that he knew? How did he learn these improvements?
Overall: R.A.B. and horcruxes are totally new. There is no way anyone reading the first five books, even obsessively, would know about them. If a reader cannot guess the overall story by deduction of given hints, then Rowling is just leading readers down the garden path instead of supplying an orchestrated lead-up to total denouement. I hope to have to eat my words when #7 comes out.
Yea! Dumbledore ends his speech with "Pip, pip!" when the wizarding world doesn't use telephones. Now, that's okay, because while inconsistent, it's minor, and it's funny. Plus I wonder if even British kids wouldn't get it these days; do Brit phones still say that? I only know it from Arthur Ransome and Rosamund Pilcher's WWII books and maybe Tintin.