Punch

Punch replaces the Melee Attack or Ranged Attack skill on the right mouse key when the default weapon is unequpped in Diablo 3.

It has an APS of 1.0 compared to the 1.2 provided by Worn Knuckles a Monk starts out with.

Despite the term "punch" implying a strike with the knuckles, the actual attacks delivered by unarmed characters may not involve that at all:
 * the Monk alternates between delivering palm strikes with her left hand and a roundhouse kicks with her right leg

As for those who do deliver punches, they can vary:
 * the Demon Hunter alternates between delivering downward crosses right the right hand and upward crosses (nearly uppercuts) with her left hand