A few changes that I can think of are:
In the book they never follow Edmund when he dissapears to find the witch's castle. In fact, after he is found to be missing one of the beaver's mentions that Edmund looks like someone who has eaten of the witch's food, (something significant in my mind). Also the children and the beavers have left long before any wolves appear and there is no chase from the beaver dam, like in the movie. The group of travelers are indeed just waking up in a sort of beaver safehouse when Father Christmas turns up. And the witch turns the fox to stone when she comes upon him and others celebrating the return of Christmas, in the book. He is not really defiant, if I remember correctly. (And the movie witch looks like frosted warmth instead of deathly pale with blood red lips. I can't imagine her being THE Jadis of the Magician's Nephew.
)
All these though, I can understand. It was like with LotR, they wanted you to be on the edge of your seat, so they heightened the action, but I liked it better in the book when you didn't know exactly how close the witch was and what she was up to.
I do wish that they had more time so that they didn't have to abbreviate the interaction between Aslan and the kids quite so much. But that too is understandable given the young age of some of the viewers.
Edit: And one last thing, the movie implied that Aslan killed the White Witch. Didn't she run off or something in the book? It has been a while since I read it and I might be confusing stories, but I thought there was a discussion about how she would return again.