why is there such a vast difference in detail between Japanese and American cartoons?

In the USA, animation is based on a style that was made popular by Disney Studios, I believe.

In Japan, animation is based on the Magna style, which was developed in Japan in the 19th century.

Different artists, different artistic traditions, produce different styles.

Western art is usually more geometric and angular than traditional Asian art, thanks to the Italian Renaissance.

the first answer went by artwork.

My answer? There is some content that will fly in Japan that just won't fly in America. Take Loli-type anime where the cast look younger than what they are, for example. If you were to put them in a suggestive situation, it would be considered child porn, even if plot clearly states that the characters are in high school (i.e. Lucky star). In Japan, it's cool. in america, hell no. Parents would never shut up.

Also, anime in Japan is a little more violent than most American cartoons. There's blood everywhere in Naruto, but when Naruto came out in the US 5 years ago, they had to censor stuff out, like konohamaru bumping into Tsunade's (large) breasts, Sasuke and Naruto's kiss, and when the blood splattered after naruto stabbed himself in the hand (the Zabuza and Haku arc).

Two different mindsets about Television.

Japan lacks many drama shos etc. so they make up in anime!