Since the discussion about yes or no regarding OS specific package managers still goes on, IMO there is one argument that is far more important than all technical or aesthetic aspects.
A language specific, but cross-platform, package manager makes publishing and using published libraries a lot simpler for /developers/. And since D wants to grow, it's extremely important to provide new developers with the most comfortable and efficient development experience, so that they also stay and get productive after the first looks. I think that package managers in Ruby, Python, JavaScript/node.js were crucial in their growth. Without them, they probably wouldn't have that rich ecosystem of libraries and tools that is available today and is one of the key reasons why so many people choose those languages. Implementing an export function to turn a D package into a variety of platform specific package formats is a possible option that could close the gap and make installing applications also comfortable for the end user.