Start on Hard
As far as difficulty levels go, Reaper of Souls offers a veritable smorgasbord of options. Many of these are unlocked through further play but in the beginning, your options are Normal and Hard. Common practice dictates using the Normal difficulty for your first playthrough. For an enjoyable and rewarding experience though, you’ll want to start with Hard.
But surely that means your first play will be more difficult than necessary? Actually it won’t. Hard increases the quantity and quality of powerful items dropping at your feet while providing a delightful boost to experience gains of 75% over Normal. This leads to a greater speed in progression and as a result more abilities and runes are afforded to you during play. Battling through the game on Normal might be significantly easier than it is on Hard but when Hard is already far from a cerebral exercise, Normal is simply dull.
It also won’t prepare you for the rest of Diablo 3. This is a game where playing for tens, perhaps hundreds of hours, isn’t a rare occurrence. Getting up to level 70 as quickly as possible is pretty much one of the most important things you can do early on in your time with Reaper of Souls.
If you’re worried about certain bosses being too difficult then there’s no need to worry. At any time you can knock the difficulty down a notch.