The Ubuntu Desktop now uses GNOME instead of Unity.
On supported systems, Wayland is now the default display server. The older display server is still available: just choose Ubuntu on Xorg from the cog on the log in screen.
GDM has replaced LightDM as the default display manager.
The login screen now uses virtual terminal 1 instead of virtual terminal 7.
Window control buttons are back on the right for the first time since 2010.
Python 2 is no longer installed by default. Python 3 has been updated to 3.6.
The Ubuntu GNOME flavor has been discontinued. If you are using Ubuntu GNOME, you will be upgraded to Ubuntu. Choose the Ubuntu session from the cog on the login screen if you would like the default Ubuntu experience.
Additionally, there are quite a few "Known issues" for Desktop that made me dare not to upgrade. I will stay with 16.04 and wait for 18.04 then.