After fiddling around for a bit today with some more virtual machines (Xubuntu and Kubuntu) then booting back to regular Ubuntu, I noticed an error message. I realized that I'd gotten out of the habit of making regular backups.
So I fired up Backintime, and ... it failed. Finally, I realized there must have been too much in the home folder (some config files may have tripped it up), so I just set it to copy my Documents and a few things I do want to save (email). Then it worked fine.
Remember, boys and girls, better to save before the crash than wish you had afterward.
Oh, and Xubuntu and Kubuntu are both very good looking, although KDE just seems to have more overhead than it's worth. Ubuntu with Unity is fine.
P.S. Later I booted the Linux Format disk and tried out Lubuntu, based on LXDE. Once again, this particular front end to a Linux distribution is both snappy and soothing. It is both familiar (bottom row with start menu and little panels to show running apps) and minimalist. These days, that sounds and feels good.