Having played some Warhammer roleplaying back in the day as well as wargaming, I always felt that the grim dark nature came from the insidious and secretive nature of chaos against the Empire. As a human society, I think it was easier to relate to the menace and darkness just below the surface. In AOS, Chaos is out in the open and a visible menace, no longer hidden, and, although humans are around, they are no longer the focus of and central to the narrative - this role being filled by the Stormcast, who are more removed from humanity and thus less easy to relate to. Unless things change quite substantially, I cannot see us returning to the Old World dark any time soon. I am looking forward to the new AOS role-playing system though.