Features:
-> change most important colors (e.g. background, window decoration, tooltips)
-> change sizes of widgets (e.g. of buttons, scrollbars or the main padding)
-> allow use of light-colored wallpapers without getting icons unreadable
-> adjust the size of start menues / panel menues / or large-toolbars
-> *new* colorize desktop icons and activate their new hover effects (requires GNOME 2.18+)
-> *new* configure your gtk engines and let your current theme be drawn by whatever gtk engine you want (requires GNOME 2.18+ or engines that support the newly introduced engine schema definitions respectively)
-> many more, of course, heh ;-)

Searched: Translators, Mockups for an intuitive user interface