When people say "Call of Duty" I would assume we now mean "Modern Warfare" part. The thing that Modern Warfare did was as RPG-style leveling up, adding new skills, etc... this made for a game with multiplayer addictiveness which built a large following behind it. It executes well to, and looks good. Isn't the best in any area, but does things that build a community behind it. And it shows that long-term interest in a game isn't based on how strong the single player is.