I generally think of "hardcore" as how ridiculously complicated things are, either in the actual game itself or things around it. Civilization is hardcore, Dungeons and Dragons is hardcore, and only the people who are serious fans, "hardcore" are willing to make the effort to play them. CoD meanwhile is a kind of pick up and play thing that people who aren't gamers in general will still play, something akin to Halo, or Madden. It just says in how it's "the most popular game in the world."
Actually I guess I look at it in terms how "geeky" something is traditionally viewed.