It's kind of treading the double-edged sword of Out of Character behavior. If written convincingly and consistently, it can be perfectly good. If, on the other hand, it just kind of happens and the author doesn't give any real reason to just go with it, then it can be very hard to sustain disbelief, especially if there are lots of references to 'In Character' behavior that highlight the disconnect.
It's probably a mistake to have this happen so quickly after the end of the last chapter of Deathly Hallows, and to have so much character turn over so quickly. Your characterization of Harry and company is about 180 degrees different from the way they're characterized in the books, which is fine in and of itself - a lot of fan fiction is about the writer's perception of the characters more than the characters themselves and that's just part and parcel of genre - but when that kind of characterization is attached to the books without any explanation it becomes much more obvious to the point of disbelief and disconnection for the reader. The behavior of Harry towards Dumbledore, Dumbledore towards Harry, the goblins towards Harry, the government of Shacklebolt behaving like Fudge, it just doesn't correspond to whats come before, which wouldn't matter if there weren't the implicit suggestion that everything that was written before happened as it was written, but there is, and even with suggestions that some extra things happened that weren't show, it's just impossible to maintain suspension of disbelief that anything you're writing could actually happen within the story. Which sounds silly when we're talking about fiction, the author can decide to make things however they want to, but in practice it doesn't work unless the author sets certain rules and abides by them, and by absorbing Rowling's work as the antecedent to this, you've absorbed some of her rules, and then proceeded to break them without convincing explanation.
It really needs a lot more time to give the characters reasons to act the way they do, or a completely alternate history to give their actions a better fitting context, but in the milieu they're in now, it just doesn't work.
The stuff you're good at - especially the Harry/Hermione/Luna interactions - are still good. Most of the character work is fine in and of itself, except that I can't believe in it.