If no path statement exists in /etc/environment MDM fails with messages in .xsession-errors that lead me nowhere with Google searches.
Something like "Error in MDM 5: exec; Init not found." I could press ctrl+alt+F2, login and issue startx and get the desktop, but Cron was not around along with many other things I rely on. By trial and error I found the problem.
It turns out that if I have no path assignment in /etc/environment rather relying on it being set in some file * in ~/.kde/env/*.sh, e.g. /home/rob/.kde/env/path.sh then MDM fails with fairly useless error messages. If I place a valid path in /etc/environment everything works and my variable expansion takes place in path.sh (variable expansion cannot take place in /etc/environment). This behavior is different than in LM 15 KDE. In LM 15 KDE my path statement in /etc/environment was commented out and path assignment took place in my path.sh file. While not necessarily a bug the behavior in LM 16 KDE is certainly different and not documented. FWIW I migrated my home directories via sudo cp -r -p -v from my LM 15 KDE partition.
It took me 3 days to track this thing down and solve it (again). It took me 3 days only b/c I am deceptively slow. ;> The upside is I learned something. That's what I like about Linux Mint and linux in general; you get to learn things that most people could care less about.
I just thought I would mention it in case others are trying to do things with the convoluted mess of processing environment variables and setting paths. I think the mess is due to the various places unix came from (UCal/Berkely, Bell Labs, DoD, ...) and the need/desire to acommodate all approaches to minimize forks.
For those interested here is a link to an excellent explanation of environment variables:https://help.ubuntu.com/community/EnvironmentVariables
I will post this in the comments section on the Petra KDE Release annoucement as well since I sorta complained there.