There's more than two changes in Mutter 3.1.90 compared to the previous 3.2 development build, but the changes catching the most attention are mentioned below.
Unredirect override-redirect fullscreen windows, such as full-screen 3D games to avoid any performance impact [Adel; #597014]
The bug in question is GNOME BugZilla #597014, which is about un-redirecting full-screen windows to avoid the performance penalty when running full-screen OpenGL games, etc. This bug has been open since 2009. Un-redirecting full-screen windows is similar to what's done by KDE and KWin. Back in May I shared benchmarks of the GNOME Shell / Compiz / Kwin to illustrate this problem.
Extend the draggable portion of window borders outside the visible frame for easy resizing with thin orders. (New draggable_border_width GConf key controls the total width of visible and invisible borders.) [Jasper; #644930]
And now it should be easier to resize windows with the GNOME Shell / Mutter as the drag-able area is greater than the visible size of the window itself.
More changes for Mutter 3.1.90 are mentioned in this mailing list message.
GNOME Shell 3.1.90 is also interesting. Improving accessibility, it adds an on-screen Cairbou-based keyboard, allows people searching in the overview using libfolks, supports displaying images in notifications (such as album cover-art for music), better Telepathy integration, improvements to the extension system, visual tweaks, and much more. See the GNOME Shell release announcement.
See the GNOME 3.2 articles on Phoronix to find out more about what you can expect out of this next GNOME desktop release. Among them is the HTML5 back-end for GTK+ 3.2, accessibility improvements, colord is an external dependency, social improvements, improved support for GNOME on tablets and other touch devices, and various annoying bug-fixes.
Look for the official GNOME 3.2.0 release on the 28th of September.