Not difficult at all. It's really only challenging at endgame when you're trying to get through the stronger rifts, but it's all about builds and equipment you find. There's plenty of difficulty levels, think about 15 or so now, that doesn't include the endgame rift difficulty either. You can play however you want. Harder difficulties get more frequent drops, more money and experience. After you hit Max level you get Paragon levels that still increase your stats so there's always something to do and gives you that little bit to move up difficulties. Honestly one of my favorite games ever. I've just played it so much I'm burnt out on it. You should definitely pick it up.