Use of standard GTK+ tabs in preferences
There has been much work on Gajim to make it conform to the HIG.
While the HIG doesn't disallow icons on tabbed notebooks it is discurraged to do this and so far Gajim is the onl application I know which uses this. I propose to change the preferences tabs to look more like this example from the HIG:
http://developer.gnome.org/projects/gup/hig/draft_hig_new/images/controls-notebook.png
This would not only be more consistent with the rest of the GNOME desktop (and I think KDE also) but it will solve a few usability issues (for example the icon used for "general" is normally used for centering text, the icon for "chat" is "document new"... those icons don't really fit at all)