1. It's very very hard to find out what's intuitive, yet powerful to use. Unlike implementing some algorithms, creating a good UI requires feedback from users.
2. It takes a lot of time and you frequently have to start over once you find out that what you thought works well, doesn't work well for others.
3. From anecdotical experience, I'd say that user interface design isn't what open source developers are interested in. It's a distraction from what they actually want to work on.
Personally, I find Inkscapes UI okay to use. Blender and Gimp, on the other hand, are a horrible, unintuitive mess. Whenever I need to do some image manipulation, I try to get by with Irfan View and Inkscape (even for raster graphics) as much as possible, just to avoid having to mess around with Gimp.
In the case of Blender once you get the hang of it, you'll feel more confortable than with other similar tools that do the same. I would say its kinda like vim.
GIMP is a mixed bag though. For the chrome of the application they definitely need more work, better icons, better toolbars, organisation of the UI etc. However, their on-screen tools (selection tool, path tool, gradients...) I find the GIMP UI much superior to all other programs I have tried.
It has nowhere near as many features though. I think part of the UI problem is handling a huge number of tools and features like in Inkscape and Blender.
2. It takes a lot of time and you frequently have to start over once you find out that what you thought works well, doesn't work well for others.
3. From anecdotical experience, I'd say that user interface design isn't what open source developers are interested in. It's a distraction from what they actually want to work on.
Personally, I find Inkscapes UI okay to use. Blender and Gimp, on the other hand, are a horrible, unintuitive mess. Whenever I need to do some image manipulation, I try to get by with Irfan View and Inkscape (even for raster graphics) as much as possible, just to avoid having to mess around with Gimp.