I mean that Mir will not replace X until 14.10, so the 14.04 LTS release will still use X by default. A lot of people have been worried about Mir being launched as the default desktop display server in an LTS, and when I spoke about it with Rick, he said that he wasn't planning for Mir to be the default in 14.04, and the reason why was that it was changing rapidly still, and the Mir they could put in 14.04 would likely not have stopped changing rapidly yet, but would have to be supported in a bugfix-only fashion for years after 2014.