Today Khaled Hosny released XeTeX 0.9999.0, the first beta version of the 0.9999 series. There are some major changes. The most notable improvement is the port from the ICU LayoutEngine to HarfBuzz.
HarfBuzz is an OpenType text shaping engine, which is under active development and offers a much wider OpenType support. This step also fixes a number of known bugs.
ICU worked well, but development has stopped, so that change is an important step forward for XeTeX.
Good news for Mac users: XeTeX got 64-bit support on the Mac. AAT font layout and font search has been ported from the deprecated ATS/ATSUI to Core Text.
There are further changes and fixes, for example now TrueType and OpenType fonts are preferred over Type1 fonts if the font name is the same, and bitmap only fonts will be rejected. You can read more in the announcement.
If you wonder about the version number: for independent free software there's no need to act up with big steps in version numbers, such as 12.0, 15.0. XeTeX version numbers are asymptotic to 1. TeX version numbers are asymptotic to Pi, MetaFont version numbers converge to e, the Euler number. Sounds like nerd fun, but it can be considered as a bow to mathematics.