Most of the improvements relate to the user interface
The latest version for GIMP brings to the table plenty of improvements touching especially on the interface.Build 2.8.4 of the application includes better names for the default filters in save and export and enhances the responsiveness of drawing tools, brush outline in particular.
On the same note the splash image code has been much simplified, which allows it to appear immediately. In this revision the text tool can start on an image even without layers.
Additional modifications available in GIMP 2.8.4 refer to Libgimp. In this respect drawable combo boxes aware of layer groups and the item width in GimpPageSelector is now fixed. On OS X the plug-in windows appear on top.
As far as plug-ins are concerned, the developer included better default values in the Drop Shadow script and repaired bugs plaguing the BMP plug-in. The full list of changes is available here.
Download GIMP for Windows
Download GIMP for Mac
Download GIMP for Linux