PCs may always have the best graphics on paper, but it'll only be accessible to those on a big budget, ie hardcore gamers. For the 360 to be out as long as it has, and release something graphically pleasing like Gears of War is incredible, the developers are just going to get more and more from the machine like stazi said as time goes on.
How are the games on the PC "far more mature"? The 360 gets a fair load of "mature" games (hell, they've even completely fucked up Bomberman making him all badass) and the PS3 and Wii will have theirs as well. I pity those who can't enjoy a game because it seems to be generally aimed at children, anyway.