Specifically, it’s built on the Testing Branch of Debian — currently the ‘Wheezy’ release (although probably not for much longer). There are pros and cons to this – if you don’t like Ubuntu, or you really prefer to start from a cleaner, more basic distribution, then being based on Debian is a big plus.
On the down side, it means that it contains a lot of things which are well behind the latest current releases, starting from the Linux kernel itself — 3.2.0 rather than the 3.7.x or 3.8.x that other recent distributions feature — and the X Window System, 1.12.4 compared to 1.13.x, and so on.
What this means is that you need to pay a bit more attention if you’re going to try LMDE, make sure your hardware is compatible and supported, and be prepared to put in a bit more effort in getting everything installed, configured and running. The fact is that pretty much describes a typical Debian user anyway, so I don’t see it as a big problem.