Here is what we read on LMDE download page:
LMDE requires a deeper knowledge and experience with Linux, dpkg and APT. Debian is a less user-friendly/desktop-ready base than Ubuntu. Expect some rough edges.
OK, this is a kind of warning. But we often read on forums that lmde is better, almost as easy and even should replace the main edition.
We should be more specific on the differences because an end-user can loose a lot of time with LMDE (like I did) and then be forced to abandon it, even if it's a fantastic distro for some users.
Here are some examples of what "rough edges" now means for me:
1) no wine package or at least it's not as easy to install
2) no access to ubuntu PPA's. That means it can be difficult to install newer version of some packages without compiling. Since it's not truly a debian testing, it can be dangerous to add a debian repo also. I had to fight a lot to install the last cairo-dock for example.
3) sound system is not as polished (especially on xfce). The integration in the panel is confusing and numerical output is harder to configure (no success here and easy on main edition).
4) system beeps that have multiple causes and are not always easy to fix.
5) a lot of messages during boot process
6) machines name on local network are not interpreted (a ssh to name.local in ubuntu-derivative will connect to "name")
7) manual installation of binary video drivers (with synaptic, not a big deal, but...)
8) localization of libreoffice and firefox is not as easy and complete.
The list could be longer or shorter, but the idea is to give a better information to the end-user when it comes the time to choose.