Damage of many monsters has been adjusted to their respective element.
For example dragons deal only fire damage including their melee damage.
This makes elemental resistance gear increasingly effective and rewards proper preparation.
Player resistances stacks additively and caps at 75% including temporary effects such as potions.
If you are unsure what element a monster is, its mostly based on common sense.
Undeads = Death damage
Dragons = Fire damage (Except Frost they do Ice damage)
Wyrm = Energy damage
Bugs = Earth damage
Heros = Physical damage
And so on..