Unless 3.0 has solved it, the gimp has a steep UI problem and a learning curve such that mass appeal on the website would be inappropriate anyway. I love it but I love it because I've been using it my whole life and know it very well. Foss in general struggles with useability due to a lot of hard to overcome problems - mainly, that by the time someone is ready to contribute to any given foss project, they're already intimately familiar with its foibles and probably have strong opinions about what UX elements are sacred cows and should not be fixed.