Because when you wait 2+ years for a new Harry Potter book that you will in all likelihood consume in less than two days, these are important considerations.
There are other craft reasons that I found Half-Blood Prince difficult, too. There aren't very many action scenes until the very end, Draco is being mysterious and absent, even Quidditch is vaguely disappointing, and there's a lot of flashbacks to Voldemort's past, which is interesting, but not exactly super exciting as say, Harry forming Dumbledore's Army.
|Confunded anyone lately?|
I also realized how obvious it was from the second chapter that Snape was going to have to kill Voldemort. (Okay, so thirteen-year-old me was a little dense when it came to story structure.) It still works, I think, because at this point Harry still doesn't understand Snape's motivations or loyalties. I'm still caught between feeling like Dumbledore was smart for withholding so much from Harry (because so much depends on him figuring things out on his own, and acting according to his character and moral compass), and getting so angry at Dumbledore because didn't we just learn that godfathers die when we withhold crucial information for no good reason? But Dumbledore's actions do hold with his character, and they underscore the fact that he's not a perfect, always-wise person. And I suppose Harry wouldn't be able to defeat Voldemort until he could see Dumbledore as the flawed person he is.
I'm not going to declare Half-Blood Prince my favorite book in the series, but I will have to bump it up above Chamber of Secrets. And now, on to the final book! I thought I knew what I was getting myself into, reading these books again. But now I'm feeling all sorts of feelings and it's overwhelming! Damn it, Harry Potter! (Just kidding, I really love it.)