According to the documentation, LMDE is based on Debian Testing, which becomes the next Debian release (currently Debian Squeeze). Testing is the version of Debian where “release testing” occurs, prior to a cutover to an actual release.
However, testing does not receive security updates the way the current release does. When Debian 5.0 (Lenny) was released, the Security Team stopped providing security updates to testing to better allocate their resources. From all indications, this “temporary” suspension continues two years later.
Linux Mint also comes in versions with various desktops, including KDE, GNOME, LXDE, and Fluxbox. Too bad there isn’t a good distribution with Window Maker as the main windowing environment – I use Window Maker when I want a low-memory environment (and GNOME or KDE is too much).