***spoilers***
I just finished the book.
I agree with Ginkasa and some other's regarding Harry's behavior. Pretty much through the whole book Harry is an arrogant jackass in my opinion. Like others have mentioned, if I were Ron or Hermione, I would have just bailed on Harry, I don't need to take that from some outcast who had no friends in the beginning. And I think Harry has forgotten this. While the rest of the students have been a part of the magical world their whole lives, Harry only learned of it when he was eleven, and now he walks around like he knows everything, and thinks so much more of himself. And he gets angry at too many people who should be the ones exploding at him. This was main thing that bothered me. Let's hope Rowling uses a spell to level his hormones in Year 6.
Sirius' death came as a surprise to me. Before this book was released we were already told that in Year 5 someone significant in Harry's life was going to die and I thought it was going to be Ron or Hermione for sure. I think it's okay though, somehow, I believe Harry will be able to contact Sirius or something.
Another disappointment was the leaving of George and Fred Weasly. These two were on my favorite character list and now that they've dropped out of Hogwarts, I don't see how Rowling will be able to include them much in the next two books, maybe something will happen, but I'm going to miss their presence in Hogwarts.
I guess that's it. Other than that, I really enjoyed reading it, and I already long for the next one.
***spoilers***