The GIMP is an extreme example, but in my experience that kind of problem seems to be epidemic among the Linux developer community - in an open development environment, programmers want to work on the stuff that interests programmers, and without some kind of additional driving force, they really have no interest in boring stuff like usability except in that it gets the program into a state where others can appreciate the cool backend stuff they put all the real work into.
This is very true. Creating a good program and creating a good user interface are two entirely different skill sets, just like writing a killer book and editing it to be readable is.
To sort of try and bring this back to our beloved amiga's, that's something that was very evident there too. With so much of the software people used being PD stuff, it wasn't uncommon to find some very rudimentary interfaces (or a command line only thing, which is garbage IMO).
For bigger programs (like openoffice or whatnot) it's usually not a big deal, since there's enough people working on it that someone will have the skill set required.
I guess I don't have much experience with GIMP specifically since Deluxe Paint is about as far as I ever made it in drawing tits on my computer. For photo editing I used some app that came with Kubuntu already, and worked decently for chopping down images. Couldn't tell you what it was called though.
edit: Let's not lose track of the fact that photoshop is 700 friggen dollars, if you don't get a discount. People never like to discuss this because /everyone/ I know that uses photoshop pirated that ****, but it's still a factor.