There is a difference between LMDE & LM17/18. Consider creation on LM17/18, it is derived from Ubuntu, which is derived from Debian. LMDE is not part of this cycle. LMDE is derived from Debian. So LMDE contains no Ubuntu-fluff. This means many software packages, wrote and designed for Ubuntu will not work with LMDE. But there are many other packages available, which will run with LMDE, but will not run with Ubuntu or LM17/18. Number-1 piece of advice for any LMDE user, do not add any ppa to you system, download and compile source-code.
This makes LMDE safer for user. Consider prospect of adding unknown ppa to any system, how do you check for bugs/security-issues, there is no way. LM17/18-users are open to million problems, which do not appear in LMDE.
Contribution by Tony Greenhalgh