MS invalidated my gamertag, I lost everything, and it was all because I didn't use hotmail enough (at all). This was pre-NXE, so all I got was an "oh well, what are you going to do about it?" from their tech after a 6 hour call that had been transferred to nearly a dozen people, who then called me back after they said they mentioned it in a staff meeting with their supervisor.
Mass Effect, my orb collecting progress in Crackdown, GTA4, a bunch of stuff, gone. I started over, and came back stronger with my newer gamertag. Losing one game ain't nothin' friend. Could be a whole lot worse.