Hmm, I was thinking similar thoughts yesterday on my walk. My thoughts below:
Advantages
* Debian focuses more on freedom than Ubuntu (as you already pointed out).
This would mean less work removing non-free software, recommendations and
occasional branding.
* Debian and Ubuntu have many similarities (e.g. Synaptic and other similar
tools).
* Debian goes through a lot of QA to make sure everything works. Ubuntu
doesn't appear do this so much (e.g. gnome-panel (fallback), unstable AbiWord
in 12.04).
* gNewSense and Trisquel might be able to use each other's work.
* Potentially better upstream relations.
Disadvantages
* Ubuntu LTS releases are supported for 5 years. Debian releases are only
supported for ~3 years.
* Difference in browser release models. Squeeze uses Iceweasel 3.5 by
default, whereas Ubuntu keeps up-to-date with Mozilla.
* Some consider Debian software to be more out-of-date. For example, Wheezy
has been frozen for 8 months. (For me, this doesn't matter, as I prefer
stability).
* Debian doesn't have PPA system (officially, anyway).
Perhaps there are other pros/cons to using Debian over Ubuntu as well.
