Interesting AU based on a fun "what if" question. What if Harry Potter wasn't at Hogwarts for the seven years that the books cover? How much would the wizarding world be different? How would Harry himself be different?
Some of the changes that fascinated me most were that (mild spoilers, found out fairly early) both Dumbledore and Sirius are alive, but that Ginny is not--because Harry wasn't there to save her. My favorite change was Draco: Harry wasn't there with him in the forbidden forest during their first year, so Draco met and faced Voldemort alone. Voldemort was moments away from possessing Draco, telling him how grateful his father would be that Draco had been given such an honor, when Hagrid comes and saves him. This scares Draco enough that he goes right to Dumbledore and requests his protection. So when the story begins nine years later, Draco is part of the Order of the Phoenix and has been one of the good guys all along--although still mistrusted because of his father.
As for Harry... he knows about magic, obviously, but has been having to sort his way through it on his own. (Another mild spoiler, also found out fairly early:) Added to that is an "accident" several years earlier in which he was blinded when Dudley through acid at him. However, being Harry, he develops an exceptional ability to "see" magic, so the only person who knows he's blind is Draco.
This whole thing was just really fascinating to read. Every character we meet may at first seem similar, but inevitably has made some choices along the way that have made them into someone totally different in this universe. Ron and Hermione, for example, were never brought together first year because Harry wasn't there to be their glue. Very, very interesting. And very fun.
The story resolves much the same: there's still the hunt for the horcruxes (but with different hunters this time...) and the horcruxes are the same and in the same places. But, the battle all unfolds differently, not least because Dumbledore is there to take part in it. So, similar enough to feel comfortable, but different enough to be fascinating and fun to read.