Well, I fixed it by going into synaptic and checking all the mint packages pertaining to desktop.
Was late, so I did not test systematically but a mint-gnome package was not loaded. It must have gone during a "sudo apt-get autoremove" command (I tried a few mail clients and had to get rid of tons of unwanted software --probably will post about mail clients !).
So it's only a hunch that "autoremove" will kick off some mint desktop related packages.
As for playing with permissions, I agree there is no more hair-pulling stuff than permissions on linux, but that goes for end-users as well as developers ! (and what's a Linux desktop for if not for playing around some, huh ?)