As much as many of you want to deny, casual games such as Nintendogs, Brain Age, Wii Fit and Wii Sports are the ones that define this generation. They led us to many other games such as Farmville, Kinectimals, Sports Champions, Dance Central, EA Sports Active, many iPhone/Android games
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As much as Call of Duty sold, it didn't really "define" this generation. It just happens to be the most popular one among many similar titles, and a natural progression from the many many other shooters that are available since several generations ago. There's nothing substantial about it that differentiate it from say Halo 2 from last generation (I am not comparing the themes or play-style of the games).
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The other group of games that define our generation are the Downloadable Titles. Similar to casual games, they are also the 'smaller' titles. Games such as Pac-Man Championship, Braid, Shadow Complex, Pixel Junk, World of Goo, Bit Trip, Limbo
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So I think overall, the smaller, casual, bit sized games are what define and impact the most this generation.