There is no difference in experience due to difficulty level, and I don't think there is an effect on loot either.
Experience from opponents does depend on your level, though. See the end of the topic
xp points per level list? spoilers possible.
Unarmed combat is more challenging than armed. One person posted awhile ago playing an unarmed character, saying Whirlwind was quite effective even without weapons, and had a decent range.
When DKS is released here I'm planing on starting a ranger, with unarmed combat use when opponents get close. I played a duel wielding warrior in D2:ED, and unarmed is more of change than any of the other melee combat expertise.