There was a recent blog post explaining what LMDE is bringing to the table: http://blog.linuxmint.com/?p=1781
. Basically, LMDE aims to bring stability to Debian testing. So you get much more recent versions of packages than with Debian stable, but the LM team tests any packages for a month before they are available as update to your system, so it will be more stable than Debian testing.
As noted in the monthly stats for July (http://blog.linuxmint.com/?p=1799
), the Ubuntu based Linux Mint will roll Gnome and LXDE versions. You can always choose to install KDE on Linux Mint 12 however.
Going forward our project will support 2 desktops with a frozen Ubuntu base (Gnome and LXDE) and 4 desktops on top of a rolling Debian base (Gnome, KDE, Xfce and Fluxbox).
Personally, I welcome this increased focus on LMDE
If you have to choose, and you want an ultra stable machine, nobody is twisting your arm to upgrade
You can stay at current version you have installed.