I would say the package dependency tree is probably the most important, defining feature of the package system. Second to that is delivery (and the fact it's so high on the list is a testament to its importance, as I can remember a time before it when we just had RPM and dpkg). Some package managers also build form source, but that's a quirk of hose those operate, not really a core requirement to delivering binaries to the end user.

Even FreeBSD's ports build system is largely defined be its dependency tree and the rules they have in place to make fetching and building easy. Whether binaries are delivered from it or source that's compiled locally is somewhat transparent to the end user, other than the extra time it takes.