Every year, one game in particular is guaranteed to sell by the truckload, regardless of any evolution in the actual gameplay itself. And that game is Call of Duty. We get the games in staggered releases - one year we get a Treyarch incarnation, and another year they release one from Infinity Ward. In essence, they are basically the same game, but with different stories, guns and abilities.
The single-player is largely without subtlety - a trait that has been absent since the first Modern Warfare - and it always ends up just being a world-hopping jaunt, in which you get to see famous landmarks get blown into a thousand chunks. Regardless of location, the environments are generally grey/brown, dull and uninteresting backdrops to all the shooty stuff. The only innovation to enter the single-player space in the franchise is the introduction of poorly judged strategy-style levels in between missions in the most recent incarnation: Black Ops 2.
This is all irrelevant, however, as it isn't the single-player that keeps people coming back year after year, it's the competitive multiplayer.
There are plenty of other shooters on the market which offer the same - although CoD arguably popularised the space, on console at least - so why is this series seemingly untouchable? I think it's down to a specific combination of features that are present.
The first, and most obvious, is the unlock progression. Each kill, assist or win all seeps in to a satisfying feedback loop, unlocking new equipment as you progress. Each gun you use gets better the more you use it, with new attachments becoming available the more points you score.
This is present in most other first-person shooters that have been released since the first Modern Warfare, just look at the Battlefield series for an example. But in CoD the rewards are much more immediate.
This system keeps you coming back for more, with the perfect loadout possibly being situated just around the next milestone. Layer over the top of this all the unlockable perks and tiers within each one, and you have a constant stream of progression - with it taking a ridiculous amount of time to unlock everything. Then, once you get to the top, and everything is unlocked, you have the option to reset it all.
What drives people to do that? Because progression is addictive, that's why. It's all part of 'operant conditioning' in which the player is being reinforced to enact the correct behaviour - killing dudes and getting bling for your weapons. Operant conditioning has been proven to work on lab rats and humans alike - with tests on rats where a button in the cage produces a random treat from a receptacle. The rats kept pressing the button, whether they were hungry or not, just to see what treat laid on the other side.
On top of this, I'm a strong believer in the power of numbers. It's the same reason we find role playing games so satisfying. As we cause damage, numbers pop up from the enemy. This is used in the recent Borderlands 2 to good effect, and a similar numbers pop up when you score a kill in Call of Duty. Not numbers indicating damage, though, but instead numbers feeding back into your score and therefore also your progression. This also applies to aesthetics, just think about the first time you played Skyrim and you came across a badass set of armour - the feels!
Watching the numbers pop up, knowing that they are feeding directly into your progression system is addictive. At the heart of it all lies one word: progression. Even outside of the unlockables, as you play the game more and more, you get better at it - like anything really - but more so in a situation like this where you're inside relatively small maps and you're subconsciously learning every inch of them as you play.
Obviously the pick up and play factor plays a big part in the game's success, forsaking the trend of a paid online component - or the need to purchase a new copy to access the features already present on the disc. The simple control system also has a hand in its fortunes, with it becoming the default layout in many clones trying to ape its achievement.
The problem that CoD now faces though, regardless of how addictive it is, is series fatigue. But with the advent of the next generation looming on the horizon, and a new game engine promised for the next iteration, Call of Duty: Ghosts, we could finally see the series take the big stride forward that we've all been waiting for. Your move EA - the king is dead. Long live the king.