GNOME Shell 3.13.4 notably brings support for scaling fonts on Wayland for HiDPI displays, so the font handling for the displays with high pixel densities is now on par with GNOME Shell's X11 support. The Wayland HiDPI font scaling for the GNOME Shell is covered by this bug report.
Other GNOME Shell 3.13.4 changes include adding gestures for various system actions, utilizing a new restart framework, handling of portal login requests, and other changes. GNOME Shell 3.13.4 changes can be learned from its mailing list post.
Meanwhile for Mutter 3.13.4 are also some notable changes. Mutter 3.13.4 fixes move/resize operations for Wayland clients, adds touch gesture support, a framework was added for restarting the compositor with nice visuals, toggling seat capabilities on VT switching, and other fixes. The Mutter 3.13.4 details are shared via this mailing list post.
These changes plus many more improvements over a six month period will be worked into the GNOME 3.14 release due out in late September.