The most glaring faux pas in the eyes of the FSF is the existence of the
nonfree and contrib repositories, hosted officially by the project. The
documentation specifically says that Debian will support the use of nonfree
software. This means that if the FSF endorsed Debian as it currently stands,
they indirectly support the use of nonfree software.
Now, the standard Debian installer will only enable "main", which is wholly
free software. (apart from one or two licenses that the FSF does not say are
"free" for one reason or another, but this is minor and is not the main issue
here). But the fact that the nonfree and contrib repositories are hosted on
official servers means that it is by association a part of the project.
The installer, while only enabling "main", will ask for certain firmware on
removable media that a driver might demand, if that firmware is nonfree, and
not included in the Debian installer. I can confirm that this behaviour
occurs when an Intel wireless card is installed, and I assume this is true
for quite a few devices. Debian was my first taste of GNU/Linux. So I
downloaded the firmware for my wireless card (in truth, this was just for
Bluetooth, and it turned out I could use WiFi without any proprietary
software, by some stroke of luck) just to be safe. Then later I read more
carefully and found out this was not the case. But it does back up the FSF's
point about the installer.
The documetation also guides people to use nonfree software, to enable the
repositories in the sources.list file, and how to get certain nonfree
programs, and drivers to work. For example the wiki shows people how to
install proprietary video drivers. I think this is a bad thing, and the FSF
is right that endorsing Debian right now would not be suitable.
There are other minor things like Debian including certain licensed software
like the original Artistic license, but this is minor compared to what I just
covered.
Personally, I do not think that including nonfree firmware for certain
drivers is so bad, as it helps enable hardware. I think this is the only
thing that can really be justified, providing it is in a separate repository,
probably not part of the project at all (i.e. unofficial, not on official
servers etc). I use Debian with "main" but I know my way around.
Regardless, the FSF understandably cannot endorse Debian.
