I think the key to Snape's loyalty... or atleast why Dumbledore believed him lays in the pensieve. In fact, I think the pensieve will hold a lot of answers. I am hoping Harry gets to use it more. It has been a carefully developed element of the story and it would be sad if it was never used again.
I was really thinking that Rowling was going to throw us for a really big loop and have the lessons not be being given by Dumbledore. My thought process on this was that at no point did Harry employ the "safety question" when in the company of Dumbledore... but as we all know I was wrong.
I think the Dumbledore painting will be very helpful to Harry.
I read another person (sorry can't remember where) say something about believing that a member of each house would be required to fight Voldermort. They said that there was obviously a Gryffindor, and Draco would be a Slytherin, and Luna a Ravenclaw. They claimed that the Hufflepuff had yet to be shown. I think there's a reason Ernie McMillan has been a somewhat developed character. I think is a Hufflepuff, but I'm too lazy to go check.
These are just some random wanderings of my mind, I'm sure I'll have a few more. Thank you for creating this community so I am not ruining the book for my friends.