I know it has M-Rated on the box.  But, I also saw the trailers for it.  And then Zero Punctuation's review of it:

http://www.escapistmagazine.com/videos/view/zero-punctuation/673-MadWorld

I am curious here.  Can someone explain WHAT about Madworld is mature?

The revelation that mature games are rarely rated mature (and vice versa) finally hit you?

Excessive gory violence???

I think they may have used the f-bomb more times then they actually said Jack's name. Plus the whole ass first through a railroad station spike might set a few parent groups off.

Mature = not suitable for little kids. I guess.

Also, I wouldn't take the ZP too seriously. I usually agree with most of what Yahtzee says or at least can see his point, even though he dislikes the Wii. His Madworld review just wasn't great, some of his points where ok, a lot weren't

It seems silly that you ask a question at the end of which you've given yourself the answer to in your first sentence. This is what makes Madworld mature:

See that M in the corner? That's what makes it rated Mature. You're also going about it all wrong. It isn't the game that is Mature, it's the audience that should be mature to handle the images and gameplay of Madworld. I still don't get why people can't grasp this concept.

For the same reason other games are mature. Blood and killing.

The ZP review was honestly the worst review I think I've seen. It didn't seem as if he even played the game. The idea that the scoring system is just some random thing that happens is just one of many things that didn't even make sense.

It's as though he just needs to make fun of stuff he makes up, so that he can attempt to be funny. Having a British accent doesn't automatically make you funny.

And as it's been stated here, it's that you have to be mature enough to handle the material in the game. The game is mature in the sense that kids shouldn't be hearing what's said, and the violence is over the top. It's not mature in the 2001: A Space Odyssey way.

Like every other game were the main purpose is violence for the sake of violence it's juvenile