What qualifies as a "mature" game?

[Kantor]

Mature means two different things. I don't like the first definition, meaning "full of sex and gore" and I honestly think the ESRB should just use age numbers like the rest of the world (12, 15, 18 and so on). Duke Nukem is extremely immature, and yet it has that M on the cover.

The second means that the game actually has mature themes. Heavy Rain is a great example of this. The game should be about relationships and discovery rather than explosions and visceral deaths. Of course, you can have both in a mature game as well - just look at Mass Effect.