Matthew has published a nice easy titled as "Why Free Software has poor usability, and how to improve it":
Many of these problems are not specific to Free Software in particular, but to volunteer software. Hobbyist proprietary programs often have poor designs for many of the same reasons. But the easiest way of getting volunteers to contribute to a program is to make it open source. And while thousands of people are now employed in developing Free Software, most of its developers are volunteers. So it’s in Free Software that we see volunteer software’s usability problems most often.