Xfce is a lightweight and distraction free desktop environment with hight customizability and stability. The next major release of Xfce, Xfce is scheduled to happen next March. The previous edition of Xfce used GTK 2, and it was expected that the next major version will use GTK 3. But some of the Xfce developers are not convinced about switching to GTK 3.
The reasons are mainly that most GTK3 code base has limited development resources and also some people hate GTK 3 (or Gnome 3 in general). Also, if Unity widgets that are written in GTK 3 are also ill maintained.
It will be interesting to see which tool-kit lands up in next Xfce release.