Diana Wynne Jones is my favorite author, and Dark Lord of Derkholm is one of my favorite books.
Addressing Dark Lord, I would have to say that it is enjoyable because of how well Jones manages to take all of the subplots, which are interesting on their own, and explain how they're related to the whole.
I'd also like to mention, as a major fantasy fan(atic), that an aspect of this book that appeals to me is also that Jones is, in some aspects, commenting on her craft. Especially relevant with the dragons, you can see some of the 'rules' that she has set up for her magical world. Any budding teenage authors should find this, and the continuance of it in Deep Secret, amazingly interesting.