Answer:
Assuming you are talking about UI or something graphical on a computer screen we'll just say the must-have features (in terms of appearance) are:
- The UI/Web/Graphical Designers basic knowledge of color theory so they don't end up putting #0000FF text on a #FF0000 background
- A good font (if there is text) that looks nice with the colors used
- A proper aesthetic to the software (I.E. Spotify's green on black modern/techno aesthetic)
