The first two didn't really apply. Sure, CoD existed before CoD4, but it wasn't very big or popular. If you said a CoD would come close to the sales of a GTA, you would have been laughed off the internet.
There wasn't much hype either. It sold 400k first week on PS3, or less than R2. On 360, it sold a million, but that was nothing compared to its LT sales...
It was word of mouth that stemmed from the pure quality of the game, that is the sole cause of its fantastic sales. It's easy to get into, enjoyable, keeps you playing, is never frustrating, controls brilliantly, doesn't lag, looks great and is accessible, with a short learning curve.
What does it have that other games don't? Basically, it does pretty much everything right. Nothing too innovative or ground-breaking, but it's polished.
(Former) Lead Moderator and (Eternal) VGC Detective 
