I just finished my first usability inspection on Kpdf. It is a really enjoyable PDF Viewer for KDE. Despite of its -compared to that commercial beast acroread- fairly early state of development KPDF is already quite usable. So the issues that were discovered mainly dealt with naming/wording/sorting of menu and toolbar entries. Another problem I found was the similarity of icons. It is difficult to differentiate "Previous Page" from "Back" visually if their toolbar icons look the same.
An old notorious friend crossed my path while going through the various actions: the menubar hiding issue. There must be an easy way to recover the menubar in every view/tool/mouse mode apart from "ctrl+m". Otherwise users may feel stupid and blame themselves for losing "that thing with the names at the top of the window". I hope there will be a KDE wide solution for the menubar issue some day, so users can transport the knowledge they gathered from one KDE application to the next.
Now I am looking forward to discussing the findings with the Kpdf developers. It's a gift from god for openusability engineers to get involved in software development at such an early stage. Usability engineers of commercial products can usually only dream of that ;-)