First up, there's better interaction between the Steam runtime and host distribution libraries. As said by the Steam release notes, the change "should let Steam work out of the box with open-source graphics drivers on modern distributions. If using an older distribution or running into problems, use STEAM_RUNTIME_PREFER_HOST_LIBRARIES=0 to revert to previous behavior."
The Steam Linux client update also now has close-to-tray behavior similar to other platforms, idle detection for switching automatically to away/snooze mode, a few fixes, updated Vulkan loader to enable Xlib support, and updated libxcb in the runtime to fix some DRI3-related crashes.
With the updated libxcb plus better interaction with the distribution's host libraries, this is an exciting update today particularly for those using the open-source Mesa/Gallium3D drivers.
Outside of Linux-specific work, this beta also has some Big Picture improvements, a variety of controller improvements, and a Steam overlay fix for Vulkan on AMD hardware.
More details on this update at SteamCommunity.com.