I have many years of experience as a software developer, have a thick skin, and welcome everyone's suggestions and dislikes. So: fire away!

Here are the major principles that guided me:
1. The Folder Tree, Folder Favorites, and Categories panels allow the user to display various sets of files. Therefore, the Folder Tree, Folder Favorites, and Categories panels are tools used by the File List panel and should be subordinate to it... that is, these panels should be displayed in a sidebar of the File List panel, not the XnView window.
2. Folder favorites are actually folders! In B3, the folder favorites even display the target favorites' subfolders. Therefore, the Folder Favorites should be in the folder tree (as a root item). They're folders!
3. The current button bar for Folders, Folder Favorites, and Categories is not screen-space efficient... there is no purpose to having a button used to display, say, the Categories (and Ratings) panel as a button 1/3 (!) of the screen wide. Too much wasted space!
4. If Folder Favorites are displayed in the Folders Tree (see above), that leaves only the Folders tree and the Categories as subordinate panels to the File List panel. Therefore, move the Folder Tree and Categories buttons to the mini-toolbar. This frees up quite a bit of screen space (no need for those very wide buttons) and makes logical sense, since the mini-toolbar is a tool of (ie. subordinate to) the File List.
5. If the Folders and Categories panels are resident in the sidebar of the File List panel, there are three possible states for their visibility: Folders showing, Categories showing, neither showing. Therefore, the mini-toolbar should have an additional button to toggle visibility of the File Lists sidebar, plus buttons to toggle between the Folders Tree and the Categories in the the File List sidebar.
OK... that's a start. See the following posts for mockups...