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.