Ultimately, Debian Policy.
The software tools and packaging formats exist only to implement that. It's Policy and the comprehensiveness of the Debian archive (60k+ packages last time I checked, a year or so back), which make it bliss. You simply rarely have to go outside it, and you can configure a system as lean or rich, or anywhere in between, as you like.
What Policy states is that the package maintainer (not the user) must or must not do, including incorporating specified, requiring manpages (though that's not a release-critical bug), so that documentation is standardised. The fact that /usr/share/doc/ has every single package installed listed in it, and they don't include the installed version number in the directory names, as RH still does, means that you can use that directory to recover or rebuild from various packaging failures. (Why would you want to do that you ask? Same reason you'd want to have a doctor or hospital nearby if you had a broken leg -- it's not so much what you'd planned on, but it's damned convenient when you find yourself in that situation.)
Joey Hess and Martin Krafft (see the latter's The Debian System) both expanded on more elements of Debian packaging vs. other systems, particularly Red Hat / RPM.
http://www.worldcat.org/title/debian-system/oclc/640083534&r...