From Jakob Nielsen, usability guru. The user's experience should be similar to all other similar applications to minimize learning curve and maximize usability.
lecture notes for the course Programming Graphical User Interfaces (PGUI). The course belongs to the curriculum of the Department of Computer Science in the University of Tampere.
How to create applications that look right, behave properly, and fit into the GNOME user interface as a whole, covers specific advice on making effective use of interface elements, and the philosophy and general design principles behind GNOME interface.