Love gimp, but what has come to drive me nuts about using it is that the icons are now all flat and colorless. It seriously slows down my workflow having to hover over each tool for a few seconds to find out what it actually does.
And this comes from somebody who has used the software pretty consistently for 20 years (Fark.com photoshop contests from my teens, woo!).
Maybe this is just a mac thing, though?
Anyway: congrats on 25 years, GIMP team! Your tool is a joy, and your work is hugely appreciated by me! Great job!