As you may know Ubuntu switches to Mir and automatically replaces the X.org server with XMir, an X server running on Mir, on Ubuntu 13.10 Saucy Salamander.
A problem with the Mir Display Server is that it does not have support for multi monitor yet, so the users will not be able to connect the laptop to a TV or projector.
Because this is an important issue, the developers are working hard to add multi-monitor support on Mir. For now, Mir only supports cloned displays (having the same image on both display and projector).
Because Mir does not have better performances than the old X.org server, or at least not yet, there are many developers that do not agree with Canonical’s decision. For example, both Kubuntu and Lubuntu decided not to use Mir on Kubuntu/Lubuntu 13.10.
If you are curious how the Mir Display Server feels and runs on your computer, follow the instructions in this article, to test it. Also, a video demonstration of Mir, created by Jono Bacon can be found here.