Now, before reading it, I had some guesses about what would happen. I'm happy to say, I saw NO spoilers! Thanks, everyone, for talking discreetly and using cut tags. And, in case you're wondering, I don't read any HP fan sites, so this was just my own wild speculation.
Guess: Dumbledore will die. Harry will either be imprisoned at the time or inadvertently the agent. Harry will witness the event.
This was the one that I got pretty much right. I know, predicting Dumbledore's death was no great feat of prestidigitation; this falls right in line with what we'd expect for this classical story mold. But I was guessing pretty close to the mark on character circumstances (Harry was imprisoned and a witness), so I'll take some points for that.
Guess: Voldemort will kill Dumbledore. Dumbledore will allow himself to be killed in order to reveal a weakness of Voldemort's to Harry. Harry will use this vulnerability in book seven.
Whiffed this one. Whether Dumbledore allowed himself to be killed is debatable given the long chain of events leading up to it; he certainly was aware he was placing himself in mortal peril, though. But that we gained insight into Draco's and Snape's characters was a surprise. I didn't expect that.
Guess: We'll learn more about Snape's other side (i.e. his role with Voldemort).
Yup. Again, an easy prediction. We learned even more than I expected!
Guess: Dudley Dursley will be the titular half-blood prince. Petunia Dursley actually has magical powers that she repressed because she so hated her sister. Dudley will join the world of wizards.
Nope. Interesting theory, though, isn't it? :)
I thought this was actually a pretty good guess. It explains why Dudley has been mentioned in so many of the books; he's obviously an antagonist with flaws that needs to be reconciled with Harry. Petunia revealed that she knows a bit about the magical world (e.g. dementors); it would also explain why Dumbledore would have leverage with her and could get her to vow to guard Harry. One possible tack would be that Dudley would align with Harry against Draco, bringing some bullying experience to Harry's aid.
Well... there's always book seven, right?