Setting, characters, and plot. The three are a delicate mix–give too much weight to one, and the whole thing topples over.
That’s why the Bartimaeus books are so incredibly good. They have all three in perfect measure. The setting is genius, gorgeous and stunningly original. The characters are not so original–but their journey through the world is, and their arcs are painfully perfect. And meanwhile, the plot progresses, driving the rest along.
It all works, and it is awesome.