Some of the specific highlights of GNOME Shell 2.31.4 include switching from the GConf to GSettings back-end with DConf, multi-head fixes, new properties and functions within the Shell Toolkit, code clean-ups, and then many other UI changes and animation improvements. The list of bugs squashed in this release is also fairly good.
The GNOME Shell 2.31.4 release announcement can be found on the GNOME mailing list.
Yesterday also marked the release of Mutter 2.31.4, but to the end-user there isn't anything too exciting within the newest release of this compositing window manager that uses the Clutter Toolkit. The Mutter release announcement is available here.
The next GNOME 3.0 development release is GNOME 2.31.4 (hence the 2.31.4 releases of most GNOME components currently updating) and is due out on the 30th of this month. The final release of GNOME 3.0 is still on track for the 29th of September.