MW: Branches out from WWII and starts to collect a larger audience. Awesome gameplay with deadly weapons, lobby system adapted from Halo.. this game grabs a lot of attention fast.

W@W:  Tricks veteran players into think Activision still cares about the OG fans by releasing a WWII-based game. Also, "Nazi Zombies" is now a number one hit with people who don't enjoy MP as much. Audience gets larger.

MW2:  The entire game is made less difficult (due to perks and killstreaks), allowing more people to pick up the game and enjoy it right from the get-go. Now the Call of Duty fan-based is bigger than ever.

BO:  With the Call of Duty franchise at its peak, it doesn't matter what Activision released because, everybody will buy it! Introduce a few new gimmicky features, and ride them out until the release of the game. Rinse and repeat with Zombies.

MW3:  A hybrid of MW, MW2, and BO. Although it may look like MW2, it plays like MW, and it has all the little gimmicky features of BO.

All I'm saying is without the previous versions of this game, we wouldn't know good from bad, and in the end MW3 is the better game because of these trial and errors. Even if it feel like we've been had multiple times, if you look at it this way, it doesn't seem so bad. Call of Duty has evolved, just in a "if it ain't broke, don't fix it" kind of way.