Visibility of system status
The system should always keep you informed about what is going on, through appropriate feedback within reasonable time. We do this using smart alert bars and prompts within our application.
Match between system and the real world
We've made things as simple as possible. Your work stacks are the stacks of paper on your desk; memos, documents. Your widgets are the little piece of information you just need to know.
Consistency and standards
We've tried to keep you in the same context at all times as you should not have to wonder whether different words, situations, or actions mean the same thing. We always follow our own conventions.
Even better than good error messages is a careful design which prevents a problem from occurring in the first place. Either eliminate error-prone conditions or check for them and present users with a confirmation option before they commit to the action.
Recognition rather than recall
Minimize the user's memory load by making objects, actions, and options visible. The user should not have to remember information from one part of the dialogue to another. Instructions for use of the system should be visible or easily retrievable whenever appropriate.
Flexibility and efficiency of use
Accelerators -- unseen by the novice user -- may often speed up the interaction for the expert user such that the system can cater to both inexperienced and experienced users. Allow users to tailor frequent actions.