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Okay since the day Call of Duty 4 was first developed, Infinity Ward wasn't afraid to take big steps.
In Call of Duty 2, there were no perks. In Call of Duty 4, however they added perks and some really new things.
Now when it comes to Modern Warfare 2, since they got so much success of changing things around for Call of Duty 4, they probably aren't afraid to take big chances of adding different features to the game such as a Alien Terrorist mode (similar to Nazi Zombie mode?), a more moderny type of gameplay, more weapons, shields (got it from magazine scans), and probably many more features than CoD 4. These people (Infinity Ward) probably care more about the gamers than more money.
The only thing Treyarch did was copy Call of Duty 4. The only things mainly different from CoD4 were better and bigger maps, vehicles, co-op, and a Nazi Zombie mode. Well there is not much of a difference except for the Nazi Zombie and vehicles. These people (Treyarch) probably care more about the money than gamers.