The Mir 0.19.0 changes can be found via this change-log page from their Bazaar repository. As can be seen from the list, Mir 0.19 is mostly about bug fixing. Additional details can be found from Mir on Launchpad.
Mir 0.19 has a number of regression fixes, client crash fixes, race conditions, and then just a few non-bug-fix enhancements. One of the enhancements is handling copy-paste between an X.Org Server and Mir.
The latest development code of Mir now has it opened up for development in the 0.20 series.