I saw it today, finally. It was all right.
I think the biggest difference is simply the source material--LOTR is a wonderfully complex, richly developed world and Narnia is a story. (And I LOVED Narnia as a kid, and didn't read LOTR until I was an adult, now it pales in comparison.)
I'm rereading the book and I'd only gotten to the beavers by the time I saw the movie, so I'll have to see if my memory lines up...but I remember being really scared by the wolves, and the White Witch. And I didn't really get a sense of menace, and spies, and evil from her. She was just the bad guy. Compare that with Sauron, who is just a giant eye, but is far scarier.
And with Peter Jackson, you had someone who LOVED the source material. This is just an adaptation. All areas of dramatic tension were blown--the Stone Table started off well, but just didn't have the emotion. And his resurrection...it was like where's Aslan, oh right there, cool!! Yay!
I would like to see Prince Caspian and Voyage of the Dawn Treader done, as those were my favorite books. They've greenlighted Prince Caspian.
Mr. Tumnus was disturbingly attractive, as was Peter. And I loved their costumes as adults--gorgeous.