The system should always keep users informed about what is going on, through appropriate feedback within reasonable time.
Jakob NielsenDeveloping fewer features allows you to conserve development resources and spend more time refining those features that users really need. Fewer features mean fewer things to confuse users, less risk of user errors, less description and documentation, and therefore simpler Help content. Removing any one feature automatically increases the usability of the remaining ones.
Jakob NielsenConsistency is one of the most powerful usability principles: when things always behave the same, users don't have to worry about what will happen. Instead, they know what will happen based on earlier experience.
Jakob Nielsen