To add to your list, aku, I would put explorability as one of my highest priorities. Basically, I want the user to feel safe trying out the features. They should never back away from using something for fear that their action might be irreversible. Most commonly, this is implemented using undo/redo commands, but other options are no doubt available e.g. automatic backups.
Also, for applications that are more process-oriented (rather than data-entry applications), I would consider implementing an interface that guide the user a bit more. Microsoft's Inductive User Interface guidelines can help here, although you need to be very careful not to overdo it, as you can easily slow the user down too much.
Finally, as with anything that includes text, make the user interface as scannable as possible. For example, if you have headings under which commands/options appear, consider putting the action word at the start, rather than a question word. The point that Maudite makes is a good example of scannability too, as the "Don't Save" button text doesn't rely on the context of the preceding paragraph.
