A completely new Qt-based user interface, which can be installed alongside the existing GTK+ and Winamp Classic interfaces. The new interface is usable today but not yet as feature-rich as the existing interfaces. In future, it will be the basis of a Mac OS X port of Audacious—stay tuned.
We have switched back to using GTK+ version 2.x by default. It has now been over three years since the release of GTK+ 3.0, and yet the “legacy” version of the toolkit provides more features relevant to Audacious, better cross-platform support, a more stable API, and lower memory usage. Audacious can still be built with GTK3 if desired, but we recommend the GTK2 variant for any desktop environment other than GNOME 3.
The source code has been converted from C99 to C++11. In addition to making the Qt port easier, the use of C++11 is already allowing more rapid development by letting the compiler work out many details that previously had to be written by hand. The plugin API has also been changed to C++
Full list of other changes, new features and fixes available here