Empathy
I believe that the single defining skill of User Experience is empathy. Without that, I would say a person is not doing UX.
There are people who can move pixels around, and can put together a set of wireframes to conduct tasks, when those things are not done not in the service of solving a problem based in how a person feels, then that is not User Experience Design.
The language is User Experience is often an abstraction of the language of people. We say "Users" rather than "People", we say "Pain Points" rather than "Problems", We say "Processes" rather than "What people do". We pour over usage heat maps and quantitative studies, and they are incredibly valuable.
These abstractions, though, have a cost, and that cost is in removing the context our work is experienced in, and without context the necessary empathy is lost. We can identify as "A Pain Point" the upload button on the bereavement section of our legal portal, and we put the fix for that on a road map, but as a User Experience Designer I've sat with a man whose wife has died and watched him crying because our system will not make it easy for him to do a simple action. This empathy should motivate us, and we should carry that empathic voice into our work, representing those people.
User Experience is about finding solutions through research and empathy. It is about finding out issues people have and then understanding how they want to feel before, during and after their attempt to solve them.
Occasionally those solutions are entirely about efficiency, but often they are not. Often, solutions are about informing people of what they are doing and how the system will react. Always, though, good User Experience is about respecting people, and that respect is a product of empathy.
There are tools I use to empathise this. The Persona remains a useful way of communicating that people are at the end of our User Experience work. The best days in User Experience Design are when I get to solve real problems as a result of extending empathy. Understanding the nature of a problem tells us what to do, understanding people tells us how to solve it.
And that is empathy.
