The thing missing in this story is conflict. Harry got a major powerup in brains, knowledge and skills while the rest of the cast are at the same level JKR made them so that a stupid protagonist would have a chance to win by luck. Also, Harry has future knowledge that is still valid, making everything infinitely easier.
By telling instead of showing, I can write a story with this premise and have the whole thing wrapped up in a single chapter. But it would be boring!