As it stands, I'm already clicking my damn arm off, and more recently I've played with several Witch Doctors who summon minions that look like enemies. In a large melee, I often end up attempting to target them before, during, and after the main battle - until I find myself clicking away at a friendly while everything else is dead. Is there some visual clue that can help me focus my Barbarian's man-wrath on the (enemy) hellspawn only?
By default, pressing 'V' on your keyboard enables enemy health bars.
I find that easier to identify enemies and you can usually tell if you're overwhelmed as well.