A picture is worth a thousand words. Illustration is the great communicator. A complex idea can be conveyed in a simple image. Making it possible to absorb large amounts of data quickly in the forms of Inforgraphics, icons, backgrounds, etc. Illustrations capture the emotional state of the subject even without the use of supportive text.
A great user interface design begins with a clear plan. Drawings, wireframes, task flows, show how your idea will come to life. Refine the details with high fidelity color mockups before going to into development. Define control behavior and take special care of operational states. Create storyboards for complex concepts and document the details.
If an application is difficult to use, people will get frustrated. One great way to improve acceptance is to conduct user testing. Make sure users represent the target audience and have them perform representitive tasks with the design. Then observe what they do and improve upon the challenging areas.