Mark Shea seems nonplussed at my opinion of Narnia, which in ten words is, "As a story, it's great. As speculative theology, it's weak."
In discussing the theology of the Chronicles of Narnia, I would start with the fundamental point that, contrary to what some say, Aslan is not a Christ-figure. In Lewis's stories, Aslan does not represent Christ, he is not an allegorization of the kingly and untame aspects of Christ that were downplayed in 1950's England.
Aslan is Christ. He is meant to be the identical Person Who became man and died for us, the very Word made flesh (albeit different flesh in a different world) we worship, the Son of God present on our altars.
From this free and conscious choice by Lewis, to make Aslan Christ rather than Christ-like, it follows that Lewis can have done this more or less well. In other words, his Aslan is as subject to criticism as Joseph Girzone's Joshua or even the Jesus in various late apocryphal writings.
It also follows that certain defenses of Aslan, Narnia, and Lewis are non-starters. You can't defend it by saying Lewis was trying to write literature, not theology; that's a false dilemma, and including a character who is supposed to be God is doing theology.
You can't fully defend it by saying Narnia's theology is speculative. Speculative theology must still be consistent with fundamental theology; the fact Lewis is making stuff up doesn't mean what he makes up can't be wrong. Pointing out that it's a speculative work defends it against charges that what happens did not or isn't going to happen; it doesn't defend against charges that what happens couldn't or wouldn't happen.
And of course you can't defend the theology of Narnia by pointing out it's intended to be, and succeeds at being, an entertaining story of imagination and wonder. That's a defense of Narnia, saving the work by sacrificing the theology. I'm quite happy to join in that defense, but it doesn't change the fact that a lot of people are still giving ill-thought defenses of the theology.
There's a certain irony that a decision that makes Narnia such a fresh and original story makes it so flawed at the same time, that what excites so many Christians about the story is its weakest part.