what is the most important aspect of design?

The most important trait a designer must have is a very, very low tolerance for bad design. Bad design creeps up into our lives when we become complacent. We just kind of accept things for how they are and really don't question it.

Any good designer has to constantly be questioning, questioning why things are the way they are and why people are doing things the way they do. That feeling has to be not only when you're at your desk building a product, but in everything. In the way your clothes are organized, the way a kitchen is organized. You have to optimize everything and constantly think about how you can make life better, reduce effort, think through certain flaws in your life.

The moment you stop feeling that discomfort, you stop designing.