why is skin color so often selected as a basis for discrimination? are other physical characteristics used in this way?
Because it is the first and most instantly obvious defining characteristic.
Others would be obesity,learning or speech impediments,dress, accents, too much or too little hair, height, odor and of course lets not forget profession (dress codes would qualify as physical characteristics?) or almost any part of a Monty Python or Larry the Cable Guy sketch.
What do Politically Correct folks laugh at, just out of curiosity? Isn't there inherently a victim an any humor?