Why Neville is so bad at magic
Theory: Neville is bad at magic because of Harry.
The one with the power to vanquish the Dark Lord approaches, born to those who have thrice defied him, born as the seventh month dies. And the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not. And either must die at the hand of the other for neither can live while the other survives. The one with the power to vanquish the Dark Lord will be born as the seventh month dies...
Sybil Trelawney is revealed to have prophesied these words at the end of Harry Potter and the Order of the Phoenix. Albus Dumbledore tells Harry that "the one" could have referred to either him or one other person before Voldemort chose Harry - and that other person was Neville Longbottom.
So, either Neville or Harry could have had the "the power to vanquish the Dark Lord". But who could find two more opposite? Harry can perform spells beyond his years - the Patronus charm, Occlumency - while Neville manages to mess everything up, barely being able to do simple spells. And yet Neville could have been the Boy Who Lived, the one who could take on Voldemort and win. Why is this?
Here is my reasons for this: before Voldemort chose to try and kill Harry Potter, both were quite ordinary little boys, because the one with the power needs to be marked by the Dark Lord as his equal. Both, however, also had the potential to become immensely powerful. When Voldemort chose Harry Potter and gave him the scar, though, Neville was effectively cut out of the prophecy, since there was now one that fit all the criteria. Which brings us to a very interesting idea - that since Harry Potter was now the subject of the prophecy, Neville Longbottom no longer had the potential to become that powerful. In fact, it's possible that the two "shared" power, and then most of Neville's power went to Harry when Harry survived Voldemort's attack, leaving Neville with very little in the way of actual magical power.
So - what do you think?