When I went to New York last December, I rushed to the Metropolitan Museum to see the exquisite small exhibition of 18th century French painter Antoine Watteau. It was the closing day of the show, and I expected it to be packed, but luckily for me, that was not the case. There was another show at the Met that Sunday that drew a big crowd: a sprawling display of American paintings telling stories of everyday life here, in this country of ours, from the late 18th to the early 20th centuries...