I still need to reply to the Maturity post, but I got kind of caught up in finishing the book off during my free time... Anyways, those'll be answered tomorrow when I wake up, as I've got to work in just a short while here.
ETA: Please don't leave spoilers in the comments. Thanks!
Okay, so I've not really read much spoilerwise, avoiding it as much as possible. I have seen the book cover and I know the name of one of the chapters is Draco's Detour, but that's it. Anyways, here's my thoughts on what's going to happen. I don't want this to be a debate or anything of the sort. It's just my thoughts on what will happen in Half Blood Prince.
I think Dumbledore is going to die after transferring his power over to Harry.
We will see less of Caps Lock Harry, but he will not be gone.
Arthur Weasley will be in consideration for Minister of Magic, though he will not gain the position. This will obviously happen after Fudge dies a mysterious death.
Ron and Hermione will go on a date together on the second Hogsmeade weekend of the school year.
Percy will attempt to make amends with his family. They will take him back, but things will be strained for a while.
The half blood prince will not be anyone we've learned of this far in canon.
Sirius will not come back. Kreacher will have either moved in with the Malfoys or his head will be mounted on the wall with the rest of his family.
Neville will display the sort of strength he had in the DA meetings inside his classes. He will have confidence in himself that he hadn't exhibited in the past. He'll still be as clumsy and forgetful as ever, but he'll be more confident.
The brains will have had some sort of effect on Ron aside from physical scarring.
Ron will be Quidditch Captain for Gryffindor. He will also have grown at least 4 more inches in height by the end of the book.
Those placed in Azkaban at the end of OotP will eventually escape or suffer under new guards, knowing its far safer for them to stay there loyal to Voldemort than to deny him in order to get out of prison. Lucius Malfoy will not act the coward this time. And if he does, he will die.
We will see more of Narcissa.
Remus will bite it. Possibly literally and figuratively.
Grawp will have some sort of purpose other than scaring centaurs away and injuring Hagrid.