. IMHO the best feature Debian has is the ability to upgrade-in-place -- so you never have to do a reinstall to keep it up-to-date unless you want to.
Indeed, I have installations of Debian that are 13 years old. They've been through multiple hardware revisions, and are now virtualized, but apt-get dist-upgrade has done the trick all this time.
However, technical achievements aside, it's Debian's policy that's the real star.