My favorite line in the first book was “Are you a witch or aren’t you?” from Ron when he was being attacked by the plant, and in this one, Hermione got to yell “Are you a wizard or aren’t you?”
By happy circumstance, I had just finished Book 1 before I started reading Book 7. It was amazing to me how many of the events in the last book referred back to the first. There were so many of them....
I was seriously struck by this when they cam to the riddle at the door of Ravenclaw's common room -- "What came first, the phoenix or the flame?" And the answer: "a circle has no beginning or end."