I don't think there is a movie that ultimately explodes with as much joy and exuberance for the happiness and perplexity of life. The last ten minutes are so unashamedly sentimental and happy they're liable to turn a tear from the most cynical soul, and I think it's James Stewart's smiling face with his ideal family surrounding him that sticks with most people.
But in the strong analysis of the film I've read, it's just as interesting from an economic side. The contrast between Potter and Bailey, while at its base is very archetypal, seems to embody so many of the ideas and discourses within the struggle for stability, especially the way it reverberates the Depression and WWII.
I think why it stands tall, not just as a Christmas film but as an American film, is because of how it magnifies what we think of as "the American character." George Bailey gives his life for his family, he works hard, he falls in love, and yet he thinks his life is worthless. We see, and he ultimately sees, that despite hardships it is his unwavering virtue that helps him survive.
This Christmas, with an economic recession on the tips of everyone's tongues and in the back of all presents, "It's a Wonderful Life" contains a poignant message for our times: it's NOT about the money, it's about who we are and how we live with what we have.