I will try to outline interesting points in a variety of systems that we can learn from. These systems may be intended for building or installing packages, or repositories for packages. I am not deeply familiar with all of the tools here, and would be interested in hearing more relevant points from someone with more knowledge. Another weakness of mine is that I don't know much about Microsoft Windows, so some good examples for Windows systems would be helpful.
The Debian GNU/Linux system is a good example of a binary distribution (meaning that packages are distributed in binary, as opposed to source code form), and its packaging system (dpkg) is somewhat similar to the more famous RPM. Debian has several other tools to help the user to install packages, most notably, apt. The Debian toolset is interesting for several reasons:
It handles dependencies extremely well. A single command can download and install a package, as well as downloading and installing all of its dependencies.
It handles updates extremely well. One command (apt-get update) checks for new versions of packages and updates a local database. Another command (apt-get dist-upgrade) downloads and installs all new versions of installed packages and any new dependencies.
There are standard commands for downloading and building packages from source. If I'm interested in hacking on a package, I can run apt-get source packagename which will download and unpack the source code for the package. The source can then be built with the standard command debuild.
The Debian Project maintains a central repository for packages, and the packaging tools offer support for using unofficial repositories as well. The central repositories include a set of servers, the autobuilders, which compile uploaded source packages for a variety of hardware architectures (see below) and make them available as binary packages. As a packager, I merely upload the source code to my package, and the autobuilders do the rest.
Currently the hardware architectures supported by Debian are Intel x86, Motorola 68k, Sun SPARC, Alpha, PowerPC, ARM, MIPS, HP PA-RISC, IA-64, S/390. Debian also runs on non-Linux systems, including GNU/Hurd, GNU/NetBSD, and GNU/FreeBSD. The package management tools also run on MacOS X under the name of the Fink project.