I believe it's been touched on a time or two, but it's a major point I think gets missed abit by the large.
When it comes to mature ( and by mature, Im not meaning the intellectual highbrow sort, I mean more adult aimed) Japan has a much much higher level of animated shows, and films than the US.
However, in the US, the ratio of more mature live action shows, and movies far out number Japans.
One ( out of many) reasons is that Japan grew fond of keeping their fantasy, or hard to duplicate in real life situations animated, for the simple reason it's easy...
another major attribute comes from a richer cultural heritage of telling stories through images as we see with ukiyo-e wood block art and kabooki.
In the US, there has been a preference for blending fantasy and reality, making something as realistic and believable as possible......this is one of the same reasons that in the UK, we tend to keep our scifi, and fantasy shows looking rather hokey in many cases... Although there are a few, most of the mature " animated" shows out of the states tend to be comedy based, again, family guy, American dad, robot chicken, southpark etc etc.
so that really says alot on the preference of medium...
in the end mature content overall equals out.