I suck at coming up with original post titles.
It struck me recently that the biggest difference between American and Japanese animation is variety. The thing is, both possess a kind of variety that the other lacks.
Note that this is all mere observation on my part as a viewer, with no actual research involved. If anyone reading this (any of the zero of you) can set me straight on any misconceptions, that'd be cool.
One of the things that sets anime (and I use "anime" solely to refer to national origin, not content, quality, or even art style -- i.e., any and all cartoons from Japan and from nowhere else) apart from American cartoons is that it's possible to create anime for practically any demographic, from little children to preteens to college students to adults, either male or female. Not that anime is particularly well-liked by Japan at large, remaining largely the purview of the geek subculture, but if there's a demographic to be found, chances are there's a cartoon aimed at them. It also comes in an extremely wide range of genres, from sci-fi to fantasy to horror to (my personal favorite) slice-of-life comedy/drama.
By contrast, American animation seems to play to very well-demarcated demographics: it's either light viewing fare for little kids or teenagers, or it's irreverent adult comedy. There's not nearly the same variety of genre or subject matter. However, American toons possess a different kind of variety: a variety of artistic, narrative, and directorial conventions. Despite its wide range of demographic appeal and subject matter, anime is practically always recognizably anime. From big round eyes and giant sweatdrops to more subtle keys like storytelling and direction, "Japanimation" has a definite set of standards. Even if you took away the visual element and just listened to the dialog in English, chances are that the discerning ear could still tell it was made in Japan. (A joke could be inserted here about the distinctively poor quality of the American dubbing industry, but I still have a big enough crush on Lisa Ortiz-as-Deedlit the Elf, so I'll let it go.) Conversely, there's no such homogeneity in American toons -- The Simpsons and South Park may both be satirical adult cartoons, but they're both very different beasts. This, I feel, is the American animation industry's greatest asset.