14.04 to 16.04: what to look out for?

Some of us will be moving from 14.04 to 16.04 having sat out 14.10, 15.04, and 15.10.

An obvious precaution would be to check that 16.04 works on one's system by ruing a Live USB.

For me, the new and shiny bits would be:
gtk 3 will be 3.18 instead of 3.10.
That will bring changes to how scrollbars work in gtk3 apps.
gtk3 apps may have client-side decorations.
Firefox and LibreOffice will now use gtk3 instead of gtk2.
LibreOffice Calc will have separate rows for sheet tabs and the horizontal scrollbar (new for those who haven't used a ppa to get newer versions of LibreOffice than the default supplied with 14.04).
LibreOffice has also "re-ordered" some of the keyboard shortcuts and that is taking some getting used to.

I don't know what "user-facing" changes systemd or the keel will bring.