I recall when I was younger many Christians were critical of CS Lewis' Narnia due to perceived pagan elements and intrusions. Recently I was a bit taken aback to realize there are contemporary 'Christian' critics who are more upset over his traditional values.
I have not read JK Rowling's books and am not really interested in them, but I've read a lot about them and have viewed some critical documentaries. Some are critical specifically of her worldview while others are critical of all forms of fantasy which incorporate magic and other dimensional worlds and thus they would be equally hostile to Lewis and Tolkien.
Rowling seemed particularly offended over the issue of Susan because to Rowling her great sin that led to her being an outcast, was that she had discovered 'sex'.
I realize that Rowling has been very controversial but some argue her works in fact are compatible with and expressive of a Christian worldview. Apparently she professes to be some kind of Christian. Of course in these days and in particular within the British context of the Churches of England and Scotland... it doesn't necessarily mean much. I know many a Mainline American Christian who's Christianity is basically comprised of the Golden Rule and a sort of works based understanding of salvation and reconciliation to a God they view in terms of agnosticism or at best Deism. The Established Church in Britain is no different. It departed Biblical Christianity long ago.
I don't know where Rowling is at but her comments seem to demonstrate a flawed and at best very shallow conception of the Christian life, sanctification and redemption from sin.
Lewis though he would seem rather worldly to many of his Evangelical fans did have a sharp and vibrant understanding of the antithesis and what it meant to be part of God's Kingdom and yet live in this world. I think Narnia itself is a sort of picture of the Kingdom. We as Christians participate in another world that does not wholly relate to or correspond with our own world. We have our own concerns, fellowship and even ideas that someone outside of the Kingdom could not possibly understand. Like an outsiders inability to get to Narnia in reality the lost cannot see the Kingdom of God.
In the Last Battle when Rillian appears to the 'Friends of Narnia' I'm reminded of the sweet fellowship that only a company of Christians can share. Of course the 'Friends' wanted to be together and share their memories, insights and experiences of a world and a kingdom that others could not understand. They were citizens of earth and England but their hearts were in Narnia or to put it in Biblical terms...Zion.
In retrospect Susan was already on a bad path. We see the first hints of a weakened faith in Prince Caspian when she is the most resistant to Lucy's insistence that Aslan is beckoning them to follow.
Susan does not appear in The Voyage of the Dawn Treader but she is referred to as having gone to America with her parents. For those who don't recall, Peter was with Professor Kirke studying for an exam, Susan went to America with her parents and Edmund and Lucy ended up with Eustace their cousin. It was through the spare bedroom in his house that they entered Narnia once more.
Susan is very grown up acting and interested in the things of the world and finally in The Last Battle we learn she is no longer a friend of Narnia. It's said that she's into clothes and makeup and those sorts of things.
Rowling is critical of these statements indicating she believes Susan has done nothing wrong. Sexuality is not a sin.
Admittedly these things are not evil in and of themselves but Lewis is demonstrating that Susan has made these things into something more, an end in and of themselves. Sex or wanting to be grown up are not inherently or intrinsically wrong but at the same time these things if not in a proper context or framed properly in the heart are indicative of worldliness and spirit that is not in submission to God's will.
Susan no longer dreamed of Narnia, she was very much at home in this world. The world's framework has become her own along with the desires and aspirations it generates.
Rowling's impoverished understanding of Christianity (which I think are probably reflected in her Potter books) misinterprets Lewis and actually tells us more about her than him.
All of this said, Lewis and Tolkien are not above criticism. The theology of Narnia is a little shaky at times but there's more going on there than people realize. Since I first read them as a child, I've re-read the Narnia Chronicles many times. Each time I revisit them I pick up on a lot more whether in the realm of philosophy, historical reference and mythological influence. Apart from the obvious commentary regarding Eustace and Jill's school or the worldview of the Telmarines, there are other more subtle and quite interesting theological and sociological comments he seems to be making and they're both interesting and often at odds with the viewpoints of many of his Evangelical fans.
Speaking of modern criticism, while Lewis is criticised for portraying Calormenes (Muslims) as cruel and demonic, I think his treatment of them is not entirely unfair. Their world contains elements of wonder and a certain majestic nobility, which I think he reflects, but ultimately from a spiritual perspective as Christians who are in a state of antithesis vis-à-vis the world... all other religions are indeed demonic.
Tolkien is probably worthy of more criticism in this regard for in Middle Earth all that is non-European is monstrous and bestial. While Tolkien has also profoundly influenced my life and thinking as the years pass I've grown both more appreciative of his project but at the same time more critical, for I see more clearly both the Roman Catholic and Sacral influences on his thinking. Lewis is by no means in the clear on these issues either but I think more profound and patently more Christian than the worldview and philosophical system reflected by Tolkien.