http://us.battle.net/d3/en/forum/topic/10490549736#3

One of our big goals with Reaper of Souls is to give players more control over their game experience, and that applies to difficulty as well. As part of that goal, we’re removing the tiered difficulty system (the one that requires you to work your way through Normal, Nightmare, Hell, and Inferno) that exists right now. We’re replacing it with something that’s more open, and (more importantly) won’t require you to play through the story multiple times if you don’t want to. 

So, the Normal > Nightmare > Hell > Inferno progression path is gone. Instead, starting in the pre-expansion patch, you’ll find that monsters will level up as you do, and they’ll gain additional affixes depending on what level range they’re in. You’ll be able to influence monster damage, health, and rewards by selecting one of nine new difficulty settings* as well, similar to the way Monster Power works currently. The result is that you should have a lot more freedom in customizing your level of challenge without worrying about multiple playthroughs of the story to get there. 

This is just a rough overview of how the new difficulties will work in Reaper of Souls. We'll have more details on the new difficulty settings in the future, so be sure to keep an eye out for them! Also, keep in mind that these could all change before we ship. 

(*Right now there’s: Normal, Hard, Expert, Master, and Torment I-V. Currently, Master unlocks at 61 and Torment unlocks at 70, but once unlocked they’re available for all characters on your account.) 

기존 보통 악몽 지옥 불지옥의 단계별 구분을 없애고,

유저의 레벨에 따라 몹들의 레벨과 능력도 스케일링되는 방향으로 가는 듯 합니다.

난이도 조절은 현행 괴물 강화와 비슷한 형태로 조정한다고 하는데 이를 9단계로 줄이고

숫자 단계가 아니라 Normal, Hard, Expert, Master, and Torment I-V 의 총 9단계로 구성하며 

Master와 Torment 난이도는 각각 61 과 70에 언락되는 형태로 하는 모양입니다.