There is no easy answer to this but if one looks at the whole realm of what art in this country HAS been, an IS, there ARE some distinctively "American" sensibilities in "American" art. One of these traits is that art should be functional. It must serve God, or serve to preserve something fleeting for posterity, or enlighten, or pursuade, or shock the viewer into some otherwise unlikely frame of mind. Another is that it must be "work". The protestant work ethic is as alive and well in American art as in most other American pursuits. A work must at least LOOK as if it took a great deal of time and effort on the part of the artist to complete in order to have any great value. Also, Americans love their art to be LITERAL. Don't make me THINK about it, just let me LOOK at it--hate it, love it, or enjoy it--I haven't got time to STUDY the damn thing. Exceptions to this exist of course, but somehow we think of them as vaguely "foreign" influences.
And finally, perhaps the MOST distinctively American trait in art is SIZE. Nowhere else in the world is the phrase "bigger is better" more readily accepted than in this country. Not satisfied with some of the enormous European canvases of the nineteenth century, Americans invented the diorama, 360-degree paintings for which special BUILDINGS had to be designed and erected just to house them. Blockbuster paintings of Civil War battles became blockbuster movies recounting the same horrors. And speaking of huge blockbusters, the British may have BUILT the Titanic, but it was left to the American's to SINK her, again and again and again in what...three, four, maybe FIVE different movies over the years? I rest my case.