Canonical had initially planned to replace the X window system with its own Mir window system in the upcoming 14.04 Long Term Support version of its Ubuntu OS, but it now appears that the change will be pushed to a later release. The announcement came at the Ubuntu Developer Summit, and it was stressed that the problem isn't with Mir itself but rather with the XMir compatibility layer that allows applications written for X to work with Mir. The fact that version 14.04 is an LTS release certainly didn't help the chances of a buggy piece of software making it in to the OS release as LTS releases are supported for five years and are generally considered to be more stable and reliable. While Mir won't be enabled by default, it will still be available in the Ubuntu repositories for users that wish to try it out at their own risk. CEO Mark Shuttleworth commented on the decision saying, "On the desktop, this will be sort of the crowning release of the Unity 7 code base. We've been working on that code pretty consistently since round about 10.04, and so we've had four very good years of continuous improvement. We can reasonably expect 14.04 to be a really fast, slick, stable, and reliable release on the desktop. That's the commitment we want to make to anyone who is deploying large volumes of Ubuntu in any large-scale environment."
Source: Ars Technica