What is the most important aspect of design
Short answer
The question before the answer.
Long answer
The foremost attribute, the most primary quality a designer must have, is openness to learn. You will not become a very sensitive designer if you say, no, that is not what I want to learn. I want to learn only what I am good at. So, openness to absorb, openness to listen to people, openness to appreciate an opinion. In every aspect of the word openness. Socially open, able to hear out somebody's argument, somebody's point of view.
And secondly, a very keen sense of observation. One of the most important abilities of a designer is to even identify a relevant problem in the environment, in the society, that needs to be solved. To find a problem which nobody has seen as a problem. That, to me, is also a very, very important key attribute. Not just to give solutions, but to even find what needs solving.