Let’s start with GNOME Shell, which is the most important part of the GNOME desktop environment for end users. The GNOME Shell 3.21.3 release plugs a memory leak, implements support for VPN (Virtual Private Network) service aliases in the network agent, and will no longer disable the suspend action when its locked.
On the other hand, Mutter 3.21.3 ships with various fixes and enhancements to the Wayland display server, which might just become standard for many GNU/Linux distributions later this year. These include better window positioning when unmaximizing through drag and drop, and support for the kill() function to force-quit unresponsive clients.