From the perspective of an ardent Catholic, and my favorite author, Flannery O'Connor. Now, in all honesty, O'Connor is specifically criticizing Catholic readers, but I think the same criticism could be levied against many Catholic writers (Tolkien and CS Lewis, I'm looking at you).
"Ideal Christianity doesn't exist, because anything the human being touches, even Christian truth, he deforms slightly in his own image...[The tendency of Catholic readers] is always toward the abstract and therefore toward allegory, thinness, and ultimately what they are looking for is apologetic fiction. The best of them think: make it look desirable because it is desirable. And the rest of them think: make it look desirable so I won't look like a fool for holding it."
-- The Habbit of Being: Letters. Edited by Sally Fitzgerald. New York, 1969. Pg 516.
I've always wondered what it is about the Lord of the Rings, or the Chronicles of Narnia, or even His Dark Materials, that I find off-putting. I think generally it is an aversion to fantasy. But when I read O'Connor's words, I realize there is another dimension to my distaste and it lies in the "thinness" and apologetic nature of the writing.
For instance, when Gandalf the Grey sacrifices himself in the first novel, it is a heart-stirring and tragic act of heroism and selflessness. And then what happens? He comes back as Gandalf the White, even better and with greater power!
Now what reasonable writer would do that? By bringing Gandalf back, Tolkien effectively removes all of the tragedy and beauty of his original sacrifice. The same can be said of the lion Aslan from the Chronicles of Narnia. Perhaps it's just a matter of personal taste, but I think both writers might have done better to drop the Jesus allegory all together so as to preserve the emotional impact of their sacrifices, even if they might have lost a few rabid Catholic readers along the way.
I believe as Flannery O'Connor believed, "The writer is only free when he can tell the reader to go jump in the lake." Then again, perhaps that's what Tolkien and Lewis are telling me.