Now here is a great set of thoughts on software usability. They are rarely followed, whether by open source/Free software types or proprietary developers. They need to be.
While you are looking at this article, be sure to follow the links to Neal Stephenson's excellent essay "In the Beginning, There Was the Command Line", which will enlighten you further. I promise this set of essays are very much worth your time, whether you develop software or simply use it.