Well, I guess this topic should also go here now
Schoelje is working in an unofficial KDE edition of LMDE. You might be interested in giving that a look, it's getting a lot of positive feedback. viewtopic.php?f=61&t=106426&start=200
Linux Mint 13 is an LTS release, based on Ubuntu 12.04. It is fully supported till April 2017, both for the Ubuntu and the Linux Mint repositories. As for Debian vs. Ubuntu base, I'd call that a personal preference as you can find proponents and opponents for each difference between the two
The LMDE FAQ gives some more background. viewtopic.php?f=197&t=91405
Cinnamon is a fork of Gnome Shell, and sits on top of Gnome 3 using all the advancements the Gnome developers bring (well, except for the Gnome Shell, though you can install that easily if you prefer it). It is being developed by the Linux Mint developers and an awesome bunch of contributing developers. MATE (http://mate-desktop.org/
) is a fork of Gnome 2. The MATE developers are actively working on fixing bugs that remained in the Gnome 2 source code after the Gnome developers stopped maintaining it.
BTW, all software developed by the Linux Mint developers is hosted on GitHub. https://github.com/linuxmint