Human psychology. Understanding users – what they need, want, and desire – is the crucial, seminal skill a user experience professional need. It influences everything – design, content, processes, features, you name it. Knowing WHO you are designing for helps you understand WHAT you need to design.
Logic and problem solving. Being able to logically think through a design and identify potential solutions for user problems allows you to build out candidate solutions. And when I say “logic” I also include an understanding of the various biases people have in this (bias and perception is a key aspect of UX).
Simplicity. Yes, simplicity is a skill. Simple is HARD. Creating a simple solution takes focus and, with reduced attention spans, is absolutely necessary in doing UI design in the 21st century.