I like that this book gives more details about Snape. In all the previous books, we're supposed to hate him. Yet because he's in the Order and Dumbledore trusts him, I started to like him a little. Plus, Alan Rickman who plays him in the movies is just outstanding and really funny.
I was totally surprised by the ending--just didn't see that coming. Yet this ending sets the tone for the chaos of the final book.