The story was like 8-10 hours, the game could have been beaten in a day really, and nothing changed in the storyline from easy, normal to hard, to inferno or wathever the last one was, it was a fun game dont get me wrong, but to me the story felt rushed, to kill the first demon king, the story paced you to it, it felt good, the other 2 where like "Ok here are the next to bad guys, kill then because reasons and GG" and i did not like that, but the game and it's mechanics where solid, the story whoever felt short in my book.
And Diablo III had nothing to do with XIV, cataclysm did whoever.