The news announcement for GIMP 2.7.3 states, "The most visible changes in 2.7.3 are the fully working single-window mode, including working session management, and the introduction of a new hybrid spinbutton/scale widget which takes less space in dockable dialogs."
The GIMP news file also notes a number of user-interface changes, improvements to GIMP's core and the libgimp library, plus a number of changes to the plug-ins.
These changes in GIMP 2.7.3 are on top of a number of other enhancements already found in the GIMP 2.7 development series, including greater usage of the GEGL library and a new brush engine.
This release doesn't incorporate the OpenCL GEGL support that we previously talked about and was worked on via this year's Google Summer of Code.
This work will eventually be made stable with the GIMP 2.8 release, which was originally set to occur around Christmas of 2010.
It then looked like GIMP 2.8 would end up being released around March of 2011. That obviously didn't happen either and the developers decided it will be done when it's done and that it won't be until after GIMP 2.8 where there's deep integration with GEGL. The last we've heard is that GIMP 2.8 may be ready by the middle of November.
There's also the GIMP 2.7 release notes to learn more about the new features, but for end-users the big item is now a completely working single-window mode.