From an interview with the Dark Materials author:
His story is a rival to the narratives put forward by two earlier Oxford writers, J.R.R. Tolkien's "The Lord of the Rings" and C.S. Lewis's "The Chronicles of Narnia". Pullman loathes the way the children in Narnia are killed in a car-crash. "I dislike his Narnia books because of the solution he offers to the great questions of human life: is there a God, what is the purpose, all that stuff, which he really does engage with pretty deeply, unlike Tolkien who doesn't touch it at all. ‘The Lord of the Rings' is essentially trivial. Narnia is essentially serious, though I don't like the answer Lewis comes up with. If I was doing it at all, I was arguing with Narnia. Tolkien is not worth arguing with."
It’s true that Lewis and Tolkien are engaged in very different projects, and the former is more didactic than the latter; that Pullman would see this as a reason to dismiss the Rings saga as “trivial” tells you a great deal about where his own fantasy saga went wrong. Being a Christian, I’m favorably inclined to Lewis’ polemical intentions, but even I can see that they sometimes step on the toes of his storytelling. Which is to say that I can see why Pullman-the-atheist would find them deeply irritating. But an appropriate response to this irritation would have been to write an “atheist’s Narnia” in which the polemic is less abrasive – and therefore more effective, perhaps – than Lewis’s Christian sallies sometimes are. More myth, in other words, and less message; more Middle-Earth, perhaps, and less Narnia. Instead, Pullman seems to have set out to take the things he hated about Lewis’ writing and recreate them, but at a heightened, more hectoring pitch. The world-building that makes The Golden Compass so compelling and fun – the panzerbjorn and the witches, the Jules Verne-meets-Tolkien landscape – is thus gradually abandoned as His Dark Materials progresses, no doubt on the grounds of its inherent “triviality,” in favor of a thudding polemic that passes well beyond Lewis and approaches the didacticism of Ayn Rand.
I also liked this bit:
Pullman says that people who are tempted to take offence should first see the film or read the books. "They'll find a story that attacks such things as cruelty, oppression, intolerance, unkindness, narrow-mindedness, and celebrates love, kindness, open-mindedness, tolerance, curiosity, human intelligence. It's very hard to disagree with those. But people will.”
Indeed. This is Atlas Shrugged in a nutshell: A style of literature-as-polemic that seeks to persuade the reader of its argument by associating those characters who share the author’s point of view with every possible virtue, and those who don’t with every possible vice. The result is a self-contained world – where Christians are all Nazis, say, or successful capitalists are all saints and geniuses – that’s persuasive so long as the reader stays immersed in it, but that can’t survive any contact or contrast with reality.