Canonical: Mir Isn't Quite Ready
Ubuntu's replacement for the X graphics system will have to wait for a later release.
Canonical's Head of Engineering Product Strategy has announced that the upcoming Ubuntu 13.10 release won't ship with the Mir graphics system enabled. Oliver Ries writes in his blog "Mir has made tremendous progress and is currently available on the Ubuntu archive for use, but there are still some outstanding quality issues that we want to resolve before we feel comfortable turning it on by default."
Mir is Canonical's entry as a next-generation replacement for the widespread yet ancient X Window graphics system. The effort to design Ubuntu for a broad range of hardware platforms (from smartphone to desktop) led to the need for a newer, more agile graphics environment. According to Ries, Mir is working well as a native system with Unity 8 on mobile devices. The technical issues are related to the XMir component, which provides an interface with X Server. For desktop systems, the eventual goal is for Ubuntu to boot using Mir if the graphics hardware supports it, and otherwise boot to X. The full implementation of this goal in the default configuration will have to wait for a later release, although Mir and XMir will still be available through the Ubuntu archives if you are interested in experimenting.