This is the spoiler for HP6. Do NOT click on the link unless you want to know everything.
It's not too late to stop reading...
OK, so you really want to know?
Severus Snape is the Half-Blood Prince. He calls himself this because his mother's surname was Prince.
Snape kills him.
No one else dies, but Bill Weasley is disfigured by a werewolf.
Tonks is in love with Lupin.
That's about it, I think.
Eeep, no, I forgot - Harry & Ginny finally get together. But after Dumbledore dies, Harry tells Ginny they can't stay together, because Voldemort kills everyone he loves.
Also, the Ron & Hermione mutual love-hate relationship is even more infuriating than usual.
Keep scrolling down for my opinions on what'll happen next...
I don't think Dumbledore's really dead. A large chunk of the story is given over to something called a Horcrux, which is a charm that someone can put part of their soul in to prevent them from complete death. It seems very likely (imo) that Dumbledore has done this, and that he WANTS Snape to kill him, so that Voldemort thinks he's truly dead. It fits in with the whole phoenix mythology.
No, I haven't read the whole book, only the ending and the first 11 chapters, but it portends. I'll be surprised if I'm wrong ;)