You can thank/blame marketing and a Michael Bay-on-steroids storyline all you want, but there's a reason people come back to Call of Duty games year after year, and it has nothing to do with zombies or sideburns.
It has to do with how the game feels.
There's a quickness to Call of Duty games that you don't see mentioned in many reviews, and don't even see mentioned that often by fans, but it is, I think, the reason people who have played (or at least been exposed to) other shooters keep playing Activision's flagship series.
It's intangible, in a way, sensed only through the impression that your iron sights "snap". That death to a bad guy comes the instant you squeeze the trigger.
You can put some of this down to the fact Call of Duty games run at 60 frames-per-second, and how that makes the game feel "sharper", but that's not the whole story.
Tech specialists Digital Foundry went a little deeper than this, and directly measured the response times between Modern Warfare 3 and the closest thing we've got to it this holiday season, Battlefield 3.
Their findings? That MW3 is over twice as fast as BF3 (50ms vs 116ms) when you're measuring the amount of time it takes from the press of a button to an action taking place on screen. While this doesn't make BF3 any less of a game—Digital Foundry point out that it's in the same ballpark as many other big-name shooters—it does explain why Call of Duty games feel so responsive.
There is of course a downside to this, namely that because Infinity Ward (and Treyarch) emphasise this speed, and go to the trouble of rendering both a few frames of footage and a gameplay input so quickly, there's little scope for graphical improvement this console generation.
DICE's scaling back to 30 frames-per-second, meanwhile, meant that it could squeeze things in like fancy lighting for Battlefield 3 that Call of Duty's ageing engine could only dream of.
So it's horses for courses, basically!
You can read the full, very interesting breakdown below.
Modern Warfare 3 vs. Battlefield 3 [DigitalFoundry]