I discovered the musical equivalent of Die Hard being people's favorite Christmas movie: Joni Mitchell's "River" is one of my favorite songs to hear around the holidays. It's not a Christmas song; it's a song about sorrow over a breakup that happens at Christmas. But like Die Hard, it's setting is clearly Christmas, and like Die Hard, it uses a Christmas tune in a minor key to set the mood (in this case, phrases from "Jingle Bells").
It's not a Christmas song, but it's such a large part of my holidays, and such a counterpoint to the prevailing sentiment, that it's easy to mistake it for one. And sung beautifully tonight, as always, at Glen Burtnik's Xmas concert in New Hope.