The "Oh" moment to me was, quite clearly, the entire chapter "The Prince's Tale". It answered so many questions, and I never imagined that Severus and Lily were more than just acquaintances. JKR is brilliant in the way that she never even hints towards it, with the possible exception of the fact that Voldemort gave Lily a choice on living or dying, and then after reading that chapter it's like "How did I not see that???" Until that chapter I was convinced Snape was evil.
It's so brilliant!"