One part of the short story which I think does work better than the movie is the part describing how Mary's life turned out due to the absence of George in it. In It's A Wonderful Life, without George Mary has become an old maid (and a librarian), and the movie treats this as if it's a terrible thing. Now, maybe it's because I'm single, but it seems to me a bit much to classify spinsterhood as a horrible tragedy. I mean, come on now, being unmarried isn't the end of the world... though it does seem to have caused Mary's hair to have been molded into an immovable bun, and ruined her eyesight, to boot:
Married Unmarried