I suppose there are some people somewhere who will argue that this is not their favourite Christmas movie, but it certainly has a very wide appeal. I'm sure most people will see it as a fantasy or a kind of up-dated Dickensian story. It is more than that. We don't need to give ourselves a headache thinking about it. The themes are universal and easily understood; love and friendship; ambition and failure; the judgement visited on the powerful; the exaltation of the meek; the importance of the individual and the value of life itself.
Heaven is a part of the story from the beginning. I have seen it many times, but I can't remember any mention of God, nor is the Church brought into it. Yet it is probably one of the most "religious" films ever made. Catholics (at least those before and around my generation) will insitinctively recognise some important Christian teachings embedded in the plot. The director, who adapted it from a stage play, brought his own particular vision to the story. Frank Capra had a Catholic upbringing and even though he more or less left the Church for a time, he never lost that "Catholic thing" that used to be cultural as well as intellectual (now it is hardly either!).
Capra had a kind of conversion experience that seems to have been related to his experiences in the war as well as to a chance meeting he had with a Christian who reminded him that we are answerable to God for the way we use our talents. It seems Capra came back to the Church by way of Christian Science - the religion of his wife. In later life he openly admitted to the Christian influence on his work. Sometimes it is obvious almost from the start as in, "You Can't Take it with You". Sometimes it only seems obvious at the end, as in "Meet John Doe". In "Mr Smith Goes to Washington" it is embedded in the story to such an extent that the final scenes have been interpreted as symbolic of the Passion of Christ.
There is an interesting essay about Capra's Catholic vision (The Catholic Vision of Frank Capra). If more people knew about this some would never watch "It's a Wonderful Life" again. Christians, however, and Catholics in particular, might gain a better appreciation of the depth and beauty of the movie.