They key principles which should be abided by.
First impressions are crucial when it comes to user interface design. Users want something that appeals to them. Would you purchase a vehicle if it wasn't visually pleasing to you? Finding a balance of design and function are very important.
When designing an user interface the target audience should be able to use the interface without instruction. If they are not able to learn from what they expected it to be doing. Adjust as needed to simplify the experience.