The idea that Harry a wizard with mediocre grades could go toe to toe with one of the greatest wizards of all time
You know how it's difficult to destroy a horcrux? Voldemort didn't realize he had created a horcrux in Harry. He also didn't realize that the Elder wand switched its allegience to Harry. The wand wasn't going to work as well against its own master, and besides, even Voldemort didn't have the magic to destroy a horcrux (prior to gaining the elder wand) - very few wizards would have even known where to start. Every horcrux in the series was destroyed by either basilisk venom or by the elder wand, which Dumbledore had (gryfindor's sword had incorporated the basilisk venom earlier in the series). Voldemort's primary failure was due to lack of information about his own situation.
This is the whole reason Harry tells everyone else that he has to battle Voldemort - he knows that he's protected by the horcrux magic.
While Dumbledore
told harry that it was his mother's love that continues to protect him, I think that's a carefully crafted lie Dumbledore tells so he doesn't have to tell Harry that he's a horcrux. His mother's love protected him the night of the attack, but the horcrux was created in Harry, and it's that horcrux that continues to protect him - not some special magic (or love) that persisted beyond his mother's death.
While the Elder wand could be used to destroy horcruxes, and could be used against its owner, I think the critical combination of both being a horcrux and its the Elder wand's owner is what protected Harry.
Ordinary wands, such as those Voldermort weilded earlier, were no mach for the horcrux inside Harry. Remember that a horcrux tries to avoid its own death - when Harry automatically protected himself during the ride of seven potters it wa actually the horcrux protecting itself, similar to the way the book, locket, and other horcruxes fought back when they were destroyed.
A horcrux is an interesting magic, in that it's vastly harder to destroy than it is to create. I'm very curious to know if Lilly knew what she was doing and actually intended for this particular outcome to occur, or if it was just her doing as much as she could, and everything else just happening because of the particular sequance of events.
Voldemort was very obviously injured from the horcrux when he attacked Harry in the forest. At that point he must have been weakened enough that with the limited power of the elderwand (against its owner) Harry could actually battle, and essentially disarm Voldemort by calling his wand to himself.
But there are still some troubling aspects to it, and I suspect that this is why they flipped things around in the movie so when the last horcrux left, Voldemort died. They needed that last battle with Harry not as a horcrux, so they had to save one to the end. I think it's a cheap play to cover up some problems, but I don't think that moviegoers who don't read the books will be bothered by it at all - and in fact I think it's because they had to dumb it down to "when the horcruxes are gone, voldemort disappears" because audiences just didn't understand.
Oh, the other big thing they really
should have had in the film was Harry fixing his own wand with the elder wand before throwing the elder wand away. It would have taken all of 5 seconds, and would have reinforced the thread that's already woven throughout, and critical to, the story - that wands and wizards are inextricably tied to each other.
And yes, I think about this a little too much.