Why? Some defensive skills, Cheng says, were made so effective that players consider them essential. Energy Armor, for example, is used by 83 percent of level 60 wizards, and War Cry's Impunity modifier is used by over 87 percent of barbarians. Nerfing the monster damage and the effectiveness of these spells is supposed to let players go to other options, rather than just walling up defense all of the time.
Cheng says there are more changes coming: A new "monster power" option to up the difficulty for higher rewards; and a tweak to the monk's One With Everything skill. We'll stay tuned for those.