Activision's had the Call of Duty line for a good long while, and it shows no sign of slowing down. The series' jump from a largely single-player, WW2-focused title to a broader base with the Modern Warfare line and the online "persistent experience" model, wherein players "level up" their accounts, changed the mechanic expected from the strategic-warfare title base.

With Modern Warfare 2, the challenge is different. Rather than establishing a new mechanic, the goal was to polish it and clean up the rough edges. Did they succeed? Well, the sales success is one thing. They've obviously leaned on the multiplayer offering to make the game successful, and single-player seems to have suffered as a result. If you're not ready to devote a ton of time to leveling up in the cooperative online world, this might be one to leave alone.