I had a few thoughts a while back about GUI's like Microsoft Windows and what principles they should follow.
* Ease of switching between applications
* Each user "command" should only have one interpretation and be read by only one program. For example, if you are scrolling a page by clicking the middle mouse button, another application shouldn't use that mouse click to do something else.
* Lack of excess flexibility and useless information. For example, you do not need to be able to position desktop or folder icons at arbitrary pixel locations. This could apply to the window paradigm itself - moving non-maximized windows around is not that fun. Another example is the dotted outline around the active widget - could be an icon, or a command button. Only used for keyboard input and worries the user whether they will accidentally activate a command.
* Uninterruptability - I.e. one program should not be able to become more prominent, taking up the screen and redirecting input commands to itself (maybe interrupting a sequence of commands intended to be received by another program/interface). An example is "splash screens" for programs. Another is the Windows Start Menu - several times when Windows has been starting up the Start Menu has disappeared while I'm trying to use it.
* Non-persistence of "high-energy" states. For example, desktop icons selection. Desktop icons should only be highlighted when the user is immediately about to do something with them. Otherwise there is a constant worry that they will accidentally rename or delete something with a single mouse click or key press.
* Lack of pointless distractions ("Unused icons in your system tray have been hidden.").
* Appearance of expensiveness of operations - for example, opening a elementary GUI feature like a menu or clicking on a tab in a tabbed interface shouldn't perform expensive calculations, load other programs or load data from slow external storage or network connections.
Updated 3rd June 2014.