The big changes made were clearly intended to give a stronger storyline, and as such, aren't unreasonable in a film adaptation of such an episodic book. What is more, the green mist theme emphasises the temptation theme which is (in my humble opinion!) the theme running through the book. Seeing the film sent me back to the book, where I realised that the ending points to this theme as well - a fish breakfast on the beach with Aslan (who is at first unrecognised); Aslan's announcement that Lucy and Edmund will not return to Narnia, but that it is not Lucy's place to ask about Eustace's future... echoes of Peter's restoration after temptation in John 21 anyone?
To be sure, Lewis' theological vision gets a little confused in translation... but to be honest, what do you expect when Hollywood tackles a novel written from a thoroughly Christian worldview? It was encouraging to see how much was retained - especially Aslan's immortal line that, "you will know me by another name in your world". When I go to see a film I expect them to misunderstand anything that is a Christian worldview, precisely because they operate from within a very different outlook. So I'm a glass half-full person in this regard, but it is certainly good to be aware of and flag up just how different C.S. Lewis' world was because the differences don't seem all that big from a superficial viewpoint.
Hollywood's 'search for the hero inside yourself' mentality was certainly (unwittingly?) inserted by the scriptwriters. Lewis would surely be aghast that the way to overcome temptation is to grit your teeth and be prepared, or that he would approve of Eustace's sudden transformation from zero to hero within a few minutes (after all, as Reepicheep explains to the poor dragon Eustace, "Extraordinary things only happen to extraordinary people"). It was sad to see Lewis' stepson (a "devout Christian" according to the Telegraph) identify the "moral core" of the book/film thus:
I have a lot of sympathy with that, but these moral virtues are certainly not the core of the Narnian Chronicles - they are side-effects. Eustace is not saved by his moral transformation, he is saved by Aslan (which is why he spends a lot less time as a dragon in the book) and therefore he is a totally different person. So don't listen to the song during the end credits (although apart from the lyrics it's a fantastic feel-good song)! The message of the Dawn Treader is not that "We can be the kings and queens of anything if we believe" or that "exactly who we are is just enough"! C.S. Lewis' great aim was not to inspire us to moral reformation or to become heroes. What was his great aim? He shows us when Aslan tells Edmund and Lucy they cannot return to Narnia:
Now that is a vision to inspire! If we spend time in Narnia and come away without seeing Christ there, we will have missed the point. Becoming a king or a queen is merely a superficial thing; knowing Christ is the most thrilling life possible. So enjoy the film, but don't listen to the siren-calls of Hollywood's offer of salvation - C.S. Lewis offers us the real thing. It may not seem like a big difference as you watch the film, but in real life it makes all the difference in the world.