I have used Debian a few times. So, my theoretical points are:
Advantages:
* Debian releases a new version only after extensive testing, unlike Ubuntu
which is more focused on a planned date.
* It does have a graphical installer called debian-installer, which is what
you'd see on any graphical install from their official iso or their
livecd/dvd. May not be as user friendly as ubiquity but it is usable by
anyone.
* They have a patched Ubuntu software-center (version 5) which does not
recommend proprietary software (although it does have debian branding). More
user friendly than Add/Remove.
* We don't have to care about upstream ubuntu bugs because they ignore a lot
of them.
* Better support for multiple architectures (mips and armel/armhf for
instance). Raspberry Pi has a plan to release a FSF-compliant version of
their chip which wouldn't work in Ubuntu.
Disadvantages:
* Hard to manage software not available on repositories. Not all PPA's work.
Would need a lot more work if we wanted to have newer software.
That's what I can think of right now.
