As this article states, Call of Duty: World at War has passed GoW2 for number of units sold on the 360. A feat made more impressive because of its multi-platform status. I think what is most interesting is that WaW beat GoW2 after a pretty bad Beta that seemed to most to be “just another WWII game” and not some blockbuster hit.
I think what actually happened was that people who have played Call of Duty 4 slowly migrated onto CoD:WaW realizing that it has very much the same hook. While CoD4 wasn’t revolutionary, it was unbeatable in terms of playability, taking me back to the days when I first picked up an [Original] Xbox controller to play Halo. In essence, the game just works and works well in the hands of any player. On the other side, GoW2 has somewhat of a niche following due to its duck and cover system. While the new gameplay style of GoW is definately a welcomed change in the community, it just isn’t as playable as Call of Duty’s Counter-Strike philosophy.
Back in the day of Counter Strike (pre-1.6 especially) games were trying to come up with new ways to play with the same coin (shooting other people). Games like Oni certainly did wonders for the genre with its fantastic combination of martial arts and gun combat but fell short because it was also somewhat hard(er) to play. Counter Strike, on the other hand, just felt right the second you started playing it. While it wasn’t the perfect game in any category and wasn’t as polished as other games it was simple and addictive. Two components that persist in Call of Duty throughout its lifetime.