There are five key elements to take into account when designing a UI:
Layout: How will the various elements of the UI be arranged on the screen?
Usability: How easy will the UI be to use? How intuitive will it be?
Aesthetics: How pleasing to the eye will the UI be?
Functionality: How well will the UI actually perform its intended functions?
Interactivity: How interactive will the UI be? How responsive will it be to user input?