Key changes in GTK+ 3.1.4 include a major visual refresh to the GtkAssistant widget, RGBA color support in GtkTextView, GtkFileChooser (finally) remembers the last opened directory, there's a GtkLockButton widget for dealing with PolicyKit and other permissions, size allocation changes, and many bug-fixes. Translation updates are also aplenty too.
That's not all either as there are also many changes to the GDK Broadway back-end too, which was merged for GTK+ 3.2. The Broadway back-end is what allows GTK applications to run in HTML5 web-browsers and has been show off to do some cool things.
With GTK+ 3.1.4, the GDK Broadway HTML5 back-end has improved key event handling, support for resize grips, make window close buttons function, and streams data over web-socket.
The GTK+ 3.1.4 release announcement can be found on the GNOME mailing list.
The GNOME 3.2 release is currently set for the 28th of September.