For some months now we have read just about everywhere, that there need to be more mature games. GTA IV has silenced those voices somewhat, but they are still there. You can hear them when you lean your head over to the Wii crowd.

What is meant by that is games rated "M for Mature" by the ESRB. This is an interesting one. Read what it says the reasons for the rating are.
"Blood" it says and "Violence". Now, when did those become part of anything "mature". You might approach those topics maturely (which most mature games don't do at all), but they are not implicitly mature.
What is meant of course is, that those are for mature audiences. Curiously, the M rating is the only one that does not say it describes the target audience, but is phrased in a way that makes it look as if it described the game rated.

This is the source of one major misunderstanding: When I talk about mature games, I mean games that are serious and meaningful, that talk about matters more important than "there's stuff, shoot it".
Granted, this is most common in M-rated games: Bioshock was about the thin line between utopia and dystopia. Prey was about free will, power and corruption.