Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Now that I've gotten all the fussing about the wrong-headed design changes last time, it's time to talk about the good news: the new features. There is no merely discussing this as "an upgrade". This is basically a reinvention of the Gimp as we knew it. Keep your hat on, as there is just too much to talk about to proceed through in any kind of orderly fashion.
The New Look - Lip-smackingly tasty. The new icon set plus the new widget integration makes Gimp look 100% better than it did before. I can't speak for other systems, but on Linux with KDE it does indeed adopt the desktop style.
Brushes - The first feature I noticed with the brush was that I could easily draw a straight line. The lines at the top were not done with holding down the shift key; instead it seems to detect when you're trying to go straight (hint: drag slowly) and keeps it that way. But only horizontally and vertically.
The brushes are indeed scalable, and in addition to pressure sensitivity, have "jitter" as well, making it so you can drag straight while applying a scattered trail. Altogether, the brush is much better and the functions are combined well in one dialog. The same attributes apply to erasers, pencils, etc.