The creators of the American Yiddish theater also provided what earlier entertainers had given to the Irish and the Germans: the immense gift of laughter.  They used gags, skits, slapstick, and wit to make fun of one another.  Romanians made fun of Hungarians.  Both made fun of Poles. All made fun of Russians.  They skewered the greenhorns, the pompous nouveau rich, the greedy landlords, the humorless goyim, the corrupt politicians; and they added something else, an attitude that forever shifted the New York mind: irony.

That is, they made jokes out of the difference between what America promised and what America actually delivered. Irony remains the essence of American humor to this day.