One of my big problems with the Harry Potter books, aside from the chronically unsympathetic protagonist (really, how do you get your lead so badly wrong?) is that they just don't make any sense. It's as if Rowling came up with some ideas ("Oooh, a dual world of magic and mundanity") without actually thinking any of it through. So there's no attempt to explain how the dual world works, why a school for wizards not only labels a full quarter of the student body as "evil" but actively encourages villainy, and so on. The stories might be good, and the characters (apart from Harry) might be compelling, but the books are just utter nonsense, and they drive me mad.
Un Lun Dun then, is a breath of fresh air (pun intended). One of China Miéville's strengths as a writer has always been his elaborate but thoughtful world-building; sometimes, as in the case of Iron Council, he forgets that we're all here for a story, but the worlds he creates are always interesting. Un Lun Dun has a good old-fashioned (only not) adventure story, a strong cast, including a great "protagonist", and best of all, it all makes wonderful sense. Yes, there's magic, and big slabs of psuedoscience, but there's an internal logic, not only to the ideas, but also to the plotting. It's a bit Neverwhere, a bit Narnia (the good bits), and even a bit Army of Darkness, and it's hugely enjoyable, a book that delights with each new invention or plot development, full of ideas that make me want to stand up and applaud. Great stuff, and I'd be happy to have Miéville write more of these, even if we never see a grown-ups' novel from him ever again.