The key to any successful marriage is compromise. While things may not always go the way you want them to, in the end, coming to an agreement helps you to achieve a greater good. The same holds true for user interface (UI) design. After all, what else is the user interface if not a marriage of form and function?
To effectively evaluate the cost and benefit of each design decision that affects the UI, you need insight across many fields, from cognitive psychology to human factors to graphic design.
Designing the UI is fundamentally an exercise in compromise—not compromise between designers and other project stakeholders (usability should never be sacrificed as a result of office politics)—but compromise between the drawbacks and benefits of design decisions. Every UI decision, from a pixel’s precise placement to the entire site’s information architecture, should be made judiciously. Careful consideration of the benefits each design decision affords and costs its users is essential. It’s the sometimes-subtle expense that many people often overlook, and every UI decision does have expense. Educated compromise across all UI decisions is essential to creating the best interface possible, and is, ironically, required if you are to avoid designing a compromised interface.