The most noticable change is to the interface, which in my personal experience was a bit clunky previously. A single-window mode now consolidates the many floating tools into one window and users can dock and move them around as needed.
Several other modifications have been added as well, including a new way to handle groups of layers and improved image exporting.
The list of changes for version 2.8 of GIMP is pretty extensive but definitely worth a look, particuarly if you're looking for a solid Photoshop alternative.