In this 4.14 cycle the main goal was to port all core components to Gtk3 (over Gtk2) and GDBus (over D-Bus GLib). Most components also received GObject Introspection support. Along the way we ended up polishing our user experience, introducing quite a few new features and improvements (read below) and fixings a boatload of bugs (read changelog).
A lot of focus seems to have been on HiDPI support, which, in 2019, is probably a good thing. Multimonitor support received quite a bit of love, too, as did other display-related things like colour profiles, display scaling, and so on.
That’s just a selection though, so be sure to read through all the changes.