A quick summary of the new features:
GEGL has been totally integrated into the backend, bringing with it support for higher bit-depth colors, non-destructive editing, and other features. The increased number of color profiles and the new single-window layout are sure to please converts from the proprietary competition. Of course, users who have become set in their multi-windowed ways (like me) can switch it back to the classic layout if they so desire.
There are also plenty of new options in terms of formats you can export to, including JPEG2000 and PDF. One downside to the newly rewritten save and export routines is that the GIMP team have chosen to separate save and export, with saving now exclusively reserved for the GIMP's native format.
Without having seen the code, I have to speculate that this has to do with the new focus on non-destructive editing and the complexity this adds to the setup. Often, non-destructive editing relies on layers, and most of the non-native image formats do not allow for layers.
Another change for layers is that GIMP 2.8 will allow them to be set up in groups, much as your desktop environment allows you to tab items in a window list. Once you get past two or three layers, a completely linear layering structure becomes confusing. Adding a second dimension to the logic allows the user to keep better tabs on things.
Two other changes that are relevant to most users are the change to Cairo for rendering and improvements in adding text to images. There are dozens of other changes to brushes and things, but I'm no illustrator. I'm afraid I'm not qualified to make a comment on them.
How to Install on Slackware64-13.37:
Since neither Pat, nor Alien Bob, nor the SlackBuild community have provided an updated package yet, early adopters will want to know how to get the GIMP up and running on their system. I haven't yet had the chance to properly test these instructions or SlackBuild file, as I spent the last two hours doing this writeup and trying to compile the sources from scratch without the build script. Some users over at LinuxQuestions are reporting issues with updated libffi.
Here is a list of the dependencies, with a link to a thorough README and a SlackBuild which was done for a pre-release version (2.7). You will have to alter the build script slightly. More to come when I've had success with these instructions.
I didn't have any luck getting the GIMP to find the version of Babl I built myself. I seem to be in pkgconfig hell, as per usual.
Also, if you use fx-foundry or other plugins, you may have to recompile them for use with the new series.
Don't forget to check out the documentation and tutorials on the GIMP's web page.
UPDATE: Users of Gnome SlackBuild have a slight advantage here: most of the upgrades that need to be performed on GIMP's dependencies have already been made by GSB. We only need to upgrade Babl, GEGL, and GTK+2.
UPDATE 2: This leads me to believe that many users who prefer DEs other than Gnome could benefit from the txz's available from GSB. You can find the appropriate packages by correlating the above linked list of dependencies with either a Slackware-13.37 or Slackware64-13.37 repository.
UPDATE 3: With the binaries or sources/SlackBuilds from Gnome SlackBuild (listed in UPDATE 2), you can install several of the dependencies. With the GTK+ team-given sources on the other three dependencies, I've begun work on SlackBuild scripts. I have successfully built the appropriate version of Babl, but both GEGL and GTK+2 seem to have errors in them. Obviously, the GIMP won't build without them. Please stand by.
Don't forget to comment with suggestions, questions, or angry ranting and follow me on Google+ for future updates.