For one thing, it's beautifully written. I found myself reading it very quietly aloud, not only to hear how the words flowed and fit together like puzzle pieces, but to form the words on my lips. It was an experience that so rarely occurs, but when it does, it's darn pleasant.
For another thing, the story is imaginative. Fairies, yes, sure, they're old hat. But these fairies are different (and also spelled differently), creepy, fantastic (in the old sense of the word), and rather peculiar.
In fact, most of the base elements of this story were not original at all, but damn, I never felt like I was reading something I'd read before.** And it is told so well and twisted just enough from what you think you know, that I was drawn in and stayed up till after 3 am, trying to finish it. (I fell asleep, though not for lack of trying not to.)
While I don't adore these characters, I do like them very much, and Arthur is probably my favorite. But I would like to see more of Ophelia, and really get to know Hettie, poor dear. And I really liked Bartholomew's character arc and decision at the end of the book.
Another thing that absolutely floored me is that not only is this a debut novel, but it was written by a teenager, who started it when he was 16 and got it published when he was 18. I admit, I am fiercely jealous of his writing ability. Props, Stefan Bachmann, props! I'm definitely going to be around for his next books! (I didn't realize until...just now, that I had read his (wonderfully creepy) short stories in The Cabinet of Curiosities: 36 Tales Brief)
** Though sometimes the writing and descriptions of London made me feel like I was reading The Amulet of Samarkand