Interactive User Interface Design

Metaphors - appeal to senses, or trigger memories. Familiar real-world UI metaphors include desktop, tapedeck, shopping cart... metaphors need not be literal objects, however. Be creative!
Consistency - of theme, colors, layout, interactive behavior
Navigation - should be a clear path forward, backward, and home
Design for Users - consider characteristics and frame of reference of intended audience
Visual Hierarchies - should clearly communicate relative importance of content elements - these can change over time, ie navigation elements that resize and relocate
Feedback and Status - controls should respond to user input with a sound, animation, or graphic change. User should be able to track their progress or location as they move throughout a project
Legibility - clear use of color, size, contrast, typography
Testing - make sure to try your interface out on as many neutral parties as possible. Observe their behavior and interview them. Respect their comments and your observations, even if it means modifying your great design!