Harry Potter Palace - The Ultimate HP website

We will learn a lot more about Voldemort in this book.
HBP will be an intense book, but with light-hearted moments.
Harry will leave Privet Drive early again, for a "much pleasanter" reason than in book 5.
Neither Harry or Voldemort (including Tom Riddle) are the Half-Blood Prince

Arthur Weasley will not be the new Minister of Magic
By the ends of books 6 and 7 "you'll have all the back story you'll need", says JK Rowling, and a prequel will not be necessary
There will be a new Minister of Magic, no more Mr. Fudge.
We will find out what happened to Hagrid's half brother Grawp. He will be a bit more controllable in book 6.
The Dursleys are in the next book, but Harry's stay with them will be the shortest yet. In book 5 he stayed for 4 weeks, so we know his stay will be less than 4 weeks.
Cho Chang will not be a romantic interest of Harry in Book 6, however there will be a "little romance" for Harry says JK.
Harry becomes even more powerful in book 6.
In books 6 and 7 we will find out why Dumbledore trusts Snape.
We will find out what happened to Wormtail AKA Peter Pettigrew
More about the animosity between Snape and Sirius will be revealed in the last 2 books.
JK has said that giving away the form of Snape's boggart and patronus says too much. We'll definitely find out what Snape's worst fear is, and it'll be important.
Hermione and Draco will NOT end up together in book 6 or 7.
We will find out what exactly Dudley saw when he looked at the dementors.
The reason Voldemort and Harry both didn't die when Voldemort tried to kill Harry, will be revealed.
We will see more of Draco's mother, Narcissa Malfoy, now that Lucius is busy.
We'll learn more about Harry's scar in the last 2 books.
Moaning Myrtle will make an appearance again, as will Tonks.
More about Voldemort's birth will be revealed, thus helping us understand why he is so evil.
Muggles begin to notice "more and more odd" occurrences in book 6, says JK.
The wizarding world is really at war in the sixth book.
Harry will tell his dearest friends about the prophecy after it sinks in to him.
We will learn more about Harry's relatives, including his grandparents (though JK says they aren't really important to the story)
We will find out what happened to Sirius's motorbike.
Book 6 will be shorter than the 5th book (or at least JK's 99% sure, but will not swear on her children's lives)
There will be no "new" nationalities of children at Hogwarts. Hogwarts is a British school, and JK says that adding foreigners for the sake of it is not in her plans.
During an interview, when JKR was asked in which Hogwarts room she'd like to be for 1 hour, she said it would be a room in which Harry has been before, but doesn't know its importance. Yet. (this could have happened in book 5, but we're unsure)
During an interview for Kids BBC, Victor Greensteet (reporter) asked Rowling if Harry would have a dragon for a pet. This is what she replied: "You can't tame a dragon, no matter what Hagrid thinks. It's simply impossible. So no. He has more common sense. He MAY have a different pet in the future, but for now, I won't say anything else". (This could have been buckbeak, but I'm sure we can expect more pets)
In the same interview for Kids BBC, the reporter asked J. K. if Harry had ever used the internet, and this is what she had to say: "No. They (the Dursleys) won't let him go near Dudley's computer, and Dudley is the only one who has a computer. They hit him if he gets too near to the keyboard. So the answer would be No. I use it a lot, but Harry doesn't. Wizards don't really need to go on the web. They have an even better way to find out what's happening in the outside world, which I think is a lot more fun than the Internet, but I'm going to keep quiet on this one."
The choice between what's right and what's easy will be a pillar of the plot in Harry's last 2 years at Hogwarts
Harry's parents' profession will be a big part of the plot.
Something HUGE will be revealed about Lily Potter.