Fred Beecher wrote what he thinks are the nine essential characteristics you must possess to make a good software user experience designer, in reaction to a comment made about skills an IA should possess. It's an excellent read for those listing qualifications and evaluating UX Designers for hire, and a great list of qualities for job-seekers to demonstrate during interviews.

  1. A Deep Understanding of Human Psychology & Research Methods
  2. Competence in the Basics of Graphic Design
  3. An Awareness of and Interest in Technology
  4. Verbal & Visual Communication Skills
  5. Moderate Familiarity with Business, Deep Familiarity with Your Business
  6. The Ability to Quickly Learn a Subject Matter Area
  7. Mediation, Facilitation, & Translation Skills
  8. Creativity & Vision
  9. Passion

Elizabeth Buie also added a familiarity with HCI literature, and undboubtedly with HCI history, which I think should be there as well. For descriptions of each characteristic read the full article