OK, I waited to see what y'all had to say, so here's mine:
Ron and Hermione become an item finally, after sparring for the last 2 books. Voldemort realizes the best way to get at Harry is to get his "family" (NOT the Dursleys) and he sets out to kill one of them. He succeeds, and kills Hermione, since he figures out that will do the most damage...killing Ron would be bad, but the most painful thing for Harry would be to see Ron devastated AND lose Hermione. He wants to goad Harry into coming after him. Harry wins in the end, and kills Voldemort...at the cost of his ability to do magic...which forces him out of the magical world and back to the world of Muggles, where he came from.
Just another note, I've studiously avoided spoilers, so this is my opinion! And if Rowling did go online and put up spoilers that were shot down by the fans, well, that would be hilarious! Tickles my sense of humour!