The idea of love. We know Harry has love- this is the one thing V doesn't have-- but how will Harry use it to defeat V? Hug him to death? Smother him with kisses? Forgive him? We've touched on the idea of love in relation to Snape but what about the other characters? Harry/anybody, Ron/Hermionie, Seriously, though, love is intangible and uncontrollable so how will it be harnessed and used?
I can blather on about this one. Since you are a librarian, I'm sure you know Madeline L'Engles
A Wrinkle in Time, which is one of my favorite books and which I am making my daughter read now and will be so upset if she doesn't like it!
Anyway, I see a similar ending for Harry. He can't love (forgive) Voldemort, just as Meg can't love IT. But he could forgive Snape, as Meg could love Charles Wallace. And in forgiving Snape, I think Harry will allow Snape to do what Harry can't - which is kill Voldemort.
Harry, with his pure and intact soul, cannot kill anyone, not even Voldemort. When he heard the prophecy, he was as aghast that he would have to kill as he was that he might
be killed.
I also think that on the way to the final confrontation with Voldemort, Harry's ability to love - and to be loved - will help him find and destroy the horcruxes.
Remember how Voldemort always prefers to work alone? He pits his followers against each other, but he doesn't trust any of them. But Harry trusts, and Harry is willing to accept help (finally - he went through a rocky patch in OoTP).
So Harry learned the lesson that Voldemort did not. You have to love, you have to risk, you have to let people care about you, or the isolation will drive you crazy.
So Harry will have Ron and Hermione with him to find the horcruxes. And he'll be able to turn to Lupin, Moody,the Weasleys, and everybody else he knows and cares about, for information and assistance.