Well, I finally finished the new Harry Potter book last night, and, as usual, J. K. Rowling didn't disappoint. Another year at Hogwart's passes with plenty of mysterious things going on to wonder about as you go through the story.
The Harry Potter books read more like mystery novels than children's books or fantasy novels, which is why I think they have such wide appeal. It's also why they seem to generate a lot of speculation and discussion, since, basically, the mystery won't be wrapped up until the final book.
The problem with mystery stories, though, is that once you know you're reading a mystery story, you expect the plot to twist and flip over on itself, and go for the most surprising ending without the reader feeling that they were cheated (meaning, the solution to the mystery could not have been predicted by the clues that led up to it). What this means is that it's very difficult for the author to get the reader to take something at face value.
And this is doubly true in the Harry Potter world where magic makes anything possible. I won't engage in spoilers, but I will say that I don't think that one of the people who dies in this book is really dead. There's some little evidence you can point to in places to support this claim, but the main reason for believing this is that, well, Rowling spent a lot of plot fuel convincing us that this person really was dead. In a mystery novel, that's a sure sign that the person is going to turn up alive and well at some later dramatic moment.
But then, Rowling knows this. I've got my suspicions about how things are going to play out, but the truth of the matter is that Rowling still has a lot of freedom - she hasn't boxed herself into a corner with the evidence and hints she's dropped to date. She's managed to sustain this mystery over six books, and it's still interesting. That's the mark of a good mystery writer.