Oh, my, where to start? The unexpected-but-oddly-logical pairing, the imaginative plot, the inventive incorporation of Tom Riddle, the complex characterizations of Helena and Cadogan, the fascinating depiction of "what it is like to be a ghost," the magic, the way the whole complicated story fits so perfectly with canon, or the elegant writing?
Well, any of them will do, because they are all so effective.
One of the many lines I liked-- She would have continued to ignore him, but the other paintings’ irritated murmurs as he ran clanking through their frames after her made it difficult for her to pretend ignorance of the commotion