Every time I hear "Rated M for Mature" on TV, I think to myself what a misnomer that is. Games that exhibit the symptoms of "mature" gameplay are often anything but. I cannot think that seeing blood spurt out of a demon's recently exposed neck-stump constitutes any concept "developed and worked out fully in one's mind". Likewise, beating hookers mercilessly hardly represents actions "fully considered and developed".
I suggest that we replace the word Mature (capital M) with Gratuitous. Because let's face it, that's what it means. Seeing comically jiggly breasts is gratuitous. Seeing your character chainsaw an alien in half or curb stomp someone is gratuitous. Let's see what Mr. Webster has to say about this idea.
Gratuitous: "not called for by the circumstances"
Mature: "characteristic of or suitable to a mature individual"
Of course, it's not like there's a bevy of games that would be joyfully re-rated under this proposal, since I can only think of one game that I'd call Mature to begin with, and it's almost nine years old.

Comments
And How!
I agree with the redefinition. Labeling games like Manhunt as "mature" has always struck me as an exercise in euphemism. One question, though: which game are you talking about at the end of the post?
Mature Game
That would be Planescape: Torment. It goes beyond the freshman level "Intro to Psych" level character analysis. The midpoint of the game, if you so choose, is a half hour long rhetorical joust with your villain.