The Chronicles of Narnia: The Lion, the Witch and the Wardrobe

It would theoretically be possible to write a plot summary which described both the original book and this movie, but it would have to be very high-level. To a Lewis purist like myself this is a mark against the movie, but I know others will feel differently. Certainly not all the changes were for the worse, but some seemed gratuitous (like assigning a line to a different character).
Some changes are entirely understandable, of course. Lewis, writing in the 1950s, could simply say that the children were evacuated from London during the war because of the Blitz and everyone knew all about that and what it was like. Now, anyone who remembers that is at least in their 60s, so spending the first 20 minutes or so depicting it was an appropriate choice.
The change that I found most jarring was the reason that the four children hide in the wardrobe. In the book, people come and visit the house on tours, and the dragon-lady who is the housekeeper has warned the children severely to keep out of the way while she shows them around. In order to avoid her unexpected appearance with a tour party they duck into the wardrobe room and hide in the wardrobe. Weak, I will grant you, but in character. What isn't in character, to me, is how in the movie Edmund breaks a window with a cricket ball, and instead of insisting that he front up and apologise, Peter and Susan run from the housekeeper along with the two younger children and hide in the wardrobe. (Pointless in any case, since the presence of the cricket ball means they will inevitably be confronted about the incident sooner or later.)
Other changes flesh out the characters (which are not very complex in the book - it's a short book, it took me less time to reread it than to watch the movie) or make the story more spectacular (the Secret Police wolves attacking the beavers' home while they flee down an underground tunnel; the whole crossing the ice/being carried away on an ice floe sequence).
All of this, however, slows the first part of the movie down, and audience attention flags a bit after a while. Then the great Lord of the Rings-style, set-piece, creature-rich battles start.
Further on fleshing out the characters, however, one problem that the book presents for someone filming it is that the impact of Aslan is described in terms of what the children thought and felt, and this can't be depicted on screen. So the magnificence and nobility and wonderfulness of Aslan is not really conveyed, which makes the impact of his death that much less - we haven't really become connected with him.
A missed trick here, and what I thought was a gratuitous change: the discussion at the Beavers' about who Aslan is was cut, and a little bit of it shifted to the end (and given to Mr Tumnus). In the book, Susan asks if Aslan isn't a man, and the Beavers explain that he is the great lion and how frightening he is to everyone, especially the Witch (who, in the movie, isn't obviously frightened of him at all - probably an improvement on the whole). "But is he safe?" "Safe? Of course he's not safe. But he's good." This characterisation would have, I believe, helped people who hadn't read the book to understand Aslan much better. I can only speculate that it was cut either to speed things up and reduce the talk in the Beavers' house, or in order to keep the "surprise" that Aslan is a lion - which nobody who had seen the advance publicity could have missed in any case.
I'll mention one final specific change: In the movie, Peter kills the wolf almost by accident and in self-defence, when the wolf leaps on his sword. He had to be brave and keep his sword up, but it was a passive bravery rather than the active bravery he shows in the book. Changing attitudes to killing wolves? Who knows.
Changed in general was the tone of the dialogue. It's quite simple in the book, and naive in a way that was probably true to children of the time but (probably quite appropriately) was updated in the movie for a more sophisticated audience. The book-Susan would never have pointed out, for example, that the Beaver "shouldn't be talking at all," but it is exactly the kind of thing that a real teenager would say. Andrew Rockell gave me a term for these: "Shrek-like moments" (Andrew Adamson, the director, also directed the Shrek movies). A classic is when Edmund says to the rearing horse he is riding, "Whoa, horsie!" and it replies, "My name is Phillip." It shifts the feel subtly, keeps popping us out of the straightforward fantasy setting and reminding us that <em>all this is actually very odd</em>.
If you check the reviews on IMDB, you will notice a lot of praise for Georgie Henley, who plays Lucy, and indeed she does extremely well. I also thought that Skandar Keynes (Edmund) did a fine acting job - actually, all the children did, but because he is a more central character, and the others are older, he is particularly deserving of mention. I imagine that being called "Skandar" would give you some insight into the bad behaviour of children, and he does a creditable job of both his earlier, sullen self and the later "reformed" Edmund. Assuming that the other movies in the series are made and that the same actors are kept for the same characters, I look forward to seeing him in Voyage of the Dawn Treader, probably my personal favourite of the Narnia books.
Tilda Swinton was great as the White Witch, striking exactly the right tone. And her castle looked just as right as she did.
The other thing that struck me about the movie, though, and even more when I re-read the book, is how <em>pagan</em> it all is. Lewis, remember, was a lecturer in medieval and Renaissance literature, whose first degree was in classics, and his last novel was a retelling of Cupid and Psyche (Till We Have Faces). Living in a time when paganism was a dead religion and neopaganism was only practiced by a tiny, obscure lunatic fringe (as opposed to the size, prominence and sometimes sanity of the fringe which now practices it), he felt free to toss in Silenus with the centaurs and fauns and nymphs and naiads and dryads. The whole story is about the defeat of winter by the returning sun - something slightly clearer in the book, where Aslan's resurrection is at the exact moment of sunrise, though his status as the Solar Lion was pretty clear in the movie also. It was a good piece of instinct on the filmmakers' part to put the witch into a chariot drawn by polar bears, because it makes her even more clearly a fearsome goddess of winter - goddesses being prone to have chariots drawn by unusual animals, such as Freya's cats.
But at the same time, Aslan is the Christ, a point which Lewis underlines for students of myth by describing in the book the creatures that were standing with Aslan when the children first see him: A bull with the head of a man (ancient Near Eastern kingship symbol), and an eagle, a pelican, a unicorn and a "great Dog", all of which are medieval symbols of Christ. (They represent resurrection and the triumphant solar hero, self-sacrifice, purity and faithfulness, respectively.) As someone who is not so much a Christopagan as an orthodox Christian with an extended symbol set, I have to like that sort of thing.
On the whole, I enjoyed the movie and would watch it again. Visually it was stunning, but I felt that it had some issues of pace and characterization and that some of the changes from the book added nothing, or even detracted.