I am totally for the darkness in storytelling, voice acting, music, graphics and the world. However what some people are longing for is the more hardcore feel of the old Diablo games, basically the feeling of being less powerful, compared to the things around you. For many people, myself include, this would be a negative thing. The aspect that makes Diablo III combat so satisfying in some cases (Uliana's set is a great example) is that you are that incredibly powerful being. This doesn't mean, that the game should be super easy, that there should be no challenge. But if we make the game so difficult, that you feel genuinely threatened by monsters, would create a great atmosphere, but as an ARPG it would not be satisfying to play.
BUT what I would LOVE, is a "true hardcore" mode where, should you choose it, the game becomes more challenging and the atmosphere more hostile. Maybe the area that your character lights up could be smaller, monsters not flash when hit, and maybe it could be it bit more zoomed in. Everything you need to make it more immersive. As an optional experience I feel this would be great.
Something similar to the modes in some horror games, where key objects don't get highlighted, hints and tutorials, maybe some helpful HUD elements are disabled.