The most notable change with the Ubuntu 21.04 desktop is now defaulting to the GNOME Shell Wayland session for supported GPU/driver configurations rather than the X.Org session. The X.Org session remains available for those that want to switch back to it or for unsupported configurations and other desktops, but finally the time has come where Ubuntu developers are comfortable enough with the support/features/performance/reliability to use it by default.
Ubuntu 21.04 is still using the GNOME 3.38 desktop by default with not having moved to GNOME 40 to allow that more time to bake. But Ubuntu 21.04 does introduce a new dark theme for its desktop.
Ubuntu 21.04 is powered by Mesa 21.0 and Linux 5.11 for offering the latest Linux hardware support. There is also a plethora of updated packages like Python 3.9 and many other latest software versions.
Downloads and learn more about Ubuntu 21.04 release announcement.