If you have one repository you've just created a monopoly. If you have multiple repositories (probably one per app) then you've just shifted the problem to installing new repositories.
What's wrong with multiple repositories? It should be easy to add new ones. There's multiple places to get debian packages, there's the main packages, non-free, backports, and other homebrew ones like mythtv etc. It's as easy as adding a line into a text file, but I'm sure the interface could be better... maybe even something integrated with the browser.
Maybe github & sourceforge has a mac repository. So you'd have the default one from apple, plus any open source apps from github & sourceforge. Individual developers who just publish code on github wouldn't need to setup their own repository. In a way, it's kind of how the ruby community distributes their packages.
