What I expect most is that everything useful is on it, and works well. This requires you (or a team of maintainers) to aggressively pursue packaging everything for it, and doing whatever that involves — emailing every author of a useful package, and so on.
For instance, the reason Debian (and its derivatives: Ubuntu etc.) is so good is that you can happily use your system without ever having to install something outside the repositories, and that everything on it is thoroughly tested. The actual features of the package manager are important, but secondary to the managed packages themselves.
