A good interface makes it easy for users to tell the computer what they want to do, for the computer to request information from the users, and for the computer to present understandable information. Clear communication between the user and the computer is the working premise of good UI design.

Good interfaces are:

Clear: A clear interface helps prevent user errors, makes important information obvious, and contributes to ease of learning and use.
Consistent: A consistent interface allows users to apply previously learned knowledge to new tasks. Effective applications are both consistent within themselves and consistent with one another.
Simple: The best interface designs are simple. Simple designs are easy to learn and to use and give the interface a consistent look. A good design requires a good balance between maximizing functionality and maintaining simplicity through progressive disclosure of information.
User-Controlled: The user, not the computer, initiates and controls all actions.
Direct: Users must see the visible cause-and-effect relationship between the actions they take and the objects on the screen. This allows users to feel that they are in charge of the computer's activities.
Forgiving: Users make mistakes. User actions should be reversible. A good interface facilitates exploration and trial and error learning.
Provide feedback: Keep the user informed and provide immediate feedback. Also, ensure that feedback is appropriate to the task.
Aesthetic: Every visual element that appears on the screen potentially competes for the user's attention. Provide an environment that is pleasant to work in and contributes to the user's understanding of the information presented.

http://uw714doc.sco.com/en/SDK_vtcl/vtclgN.style_goodui.html

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