Xbox 360 Games
Despite having a ton of amazing games, the actual list of exclusive games for the 360 is super disappointing. As someone who likes owning multiple systems based on the sort of identity their exclusives net them, its a bummer. There are the Halo games, Gears, and Rare's offerings, but outside of that there's just a few strange JRPGs, a ton of garbage Kinect games, and racing games I don't actually care about.
Don't get me wrong, its a great system, and there's a lot going for it. Its just that its identity is limited beyond "runs a lot of games better than a PS3 does, but not as good as a PC".