The Mir display server still has a long way to go
Xubuntu, an Ubuntu and Xfce-based distribution, is the first to implement and provide a working image based on XMir.Canonical is working on its Mir display server which will be implemented in the upcoming Ubuntu 13.10 (Saucy Salamander) images. Most of the developers of the other flavors haven't expressed any preferences, with the exception of KDE.
“Our good friend Unit193 has set up an ISO we can use to live-session test an XMir Xubuntu session. It is of EXTREME importance that we do as many tests as possible as the technology evolves.”
“We need to understand how our system will behave under XMir in order to make the decision later this month whether to move our standard ISOfrom X.Org to XMir,” stated Bruno Benitez on the official Xubuntu mailing list.
XMir is an implementation of X running on Mir. It's designed to work as a buffer for the developers who don't want to go straight to Mir.