I finished Harry Potter and the Half-Blood Prince last night (I spun it out for a week of evening reading pleasure) and thought I'd post some comments, thoughts, and theories.
I enjoyed the book, and rather more than the last one, because despite Dumbledore's death, this one doesn't seem as bleak in tone, possibly due to Harry coming out of his teenage sulk. I found the various crushes and pairings a bit annoying only in that they took up rather too much narrative for my liking, though I have no objections at all to any of them--they're JKR's characters to write as she likes. I did guess that Fleur would come through for Bill though; I thought there was more to her. I do hope we get a lot more of Ginny in the next book as I'd like to get to know her better and see her developed more.
Houses I don't think that Slytherin necessarily equals evil: Slughorn might enjoy living through his more successful pupils and basking in their reflected glory, but he's basically on the right side--as is Snape (more of that below). Also, we know that members of other houses can behave very badly too: Harry's father bullies Snape, the Hufflepuff who called the Quidditch game was very partisan and vicious, and Cormac McLaggen is a nasty piece of work and he's a Gryffindor. Then there's Harry himself. When Snape asks him about the potions book and calls Harry a liar and a cheat, he's quite right: Harry is both. Things are not therefore black and white--just as in RL.
Snape Ah, yes. After my initial shock (though I was expecting Dumbledore to be the one to die) I've gone back and reread some chapters. I don't think that what we and Harry see on the surface is the truth. In the first book, Snape seems bad but saves Harry's life. I noticed, even the first time I read chapter 2, that when Cissa asks Snape to vow to perform Draco's task if he can't, Snape's hand twitches as if he wants to pull it away. And when Malfoy finally confronts Dumbledore and can't do it, Dumbledore lets him (and us) know that he knew about it.
"I appreciate the difficulty of your position," said Dumbledore. "Why else do you think I have not confronted you before now? Because I knew that you would have been murdered if Lord Voldemort realised that I suspected you."The only way he'd know is if Snape told him; Harry had nothing but formless suspicions.
I think Dumbledore was dying at the start of the book from some injury that even his power could not heal. He is very insistent that Harry get him to Snape when he realises that his strength is failing.
When Snape appears on the tower, Dumbledore says his first name softly (to me a sign of friendship and closeness) and to Harry (we see everything through Harry's biased perceptions) it sounds like pleading. Perhaps it is, but I can't see Dumbledore pleading for his life. I think he's either telling Snape to do what he must, or perhaps absolving him because he knows Snape has no choice. Now, did the spell Snape actually says (Avada Kedavra) kill Dumbledore, or did Snape use something unspoken and much more merciful? Possibly Snape also did something that will allow Dumbledore to wield some power after death. Unspoken spells get a lot of mention for, I'm sure, good reason. I do think that Dumbledore really is dead though. Firstly, Snape was bound to kill him, and secondly, he's in his portrait in (now) McGonagall's study. I do wonder how much of him is there.
And finally, as Snape is escaping and parrying Harry's spells, he in effect tells Harry to learn to keep his mouth shut and his mind closed, excellent advice that Dumbledore wanted him to teach Harry.
I do like that many of the characters are so complex: Harry does shameful things (lying and cheating) and Snape, although he protects Harry and gives him advice, picks on him in class and makes his life as miserable as possible. But you don't have to like a weapon to know that it must be kept safe and in good working order.
And besides, Dumbledore trusts Snape and I don't think he's at all stupid and gullible, even if Malfoy does.
Malfoy We see a little more depth to him when he can't kill Dumbledore and obviously is deeply troubled well before that. Myrtle tells Harry that "he's sensitive, people bully him too,and he feels lonely and hasn't got anyone to talk to, and he's not afraid to show his feelings and cry." I guessed it was Malfoy straight away, but it was inevitable that Harry wouldn't have any sympathy for him though he must have remembered what Myrtle said. Yes, Malfoy attacked Harry when he came in, but though Dumbledore says that Harry has a power Voldemort hasn't--that of love--I don't think there's much danger of him redeeming Voldemort or Malfoy by the power of lurve. :-)
Professor Trelawney She seems so ineffectual apart from the prophecy 15 or so years before, but here she does predict disaster coming to the lightning-struck tower. I think she has powers she hasn't yet really accessed and we'll hear more from her.
RAB and names Snape called himself the half-blood prince after his mother Eileen Prince (interesting that he has a Muggle father like Voldemort), but there's another prince. I do love the way JKR plays with Latin (occlumens and legilimens and most spells) and other people have already mentioned Regulus Black as being RAB, but I don't think anyone I know has noticed yet that Regulus is Latin for 'prince'. For that matter, Sirius had a very appropriate name too. :-)