
I was utterly floored at the way Rowling tied everything together in Book 7. It was absolutely amazing.
If you've ever read her blogs/postings on her various websites, the reason why it ties up so nicely in Deathly Hallows is because she knew how the saga was going to end before she even started it. From what I've read, the entire series was laid out before she even wrote the first book. About the only thing that was still up in the air was certain details that would appear in either Chamber of Secrets or Half-Blood Prince (amazing, considering how far apart in time the two books are).