'Content' includes many things -- storyline, enemies, zones, and *character build options*. Being able to replay the game using an entirely different strategy is a form of additional content. That's what I love most in these type of games.
PoE has that in spades, at least through the first one or two difficulty levels. (At high levels, only a few strategies are viable, which is why the game starts to get more tiresome.) It also has enough storyline lore and enemy diversity to be consistently interesting. (I disagree with your comment about PoE exiles having no backstory, by the way. Ask the Scion about her husband sometime.)
D3 has more enemies and more storyline, but there are almost no character builds to speak of. You choose a class. Every character of that class has the exact same skills, and they all do the same thing, and if there are slightly different builds it's free and easy to switch between them. That means there are reduced strategic options, which means less replayability.