Factors of a good interface designer

Knowledge

Pattern knowledge
Trend knowledge
A desire to learn more
A desire to explore good design

Experience

A sense of where to start/what will work
Good taste
Good judge of your own design and others’
Comfortable working in many styles
Willing to have a strong opinion

Creativity

Novel ideas
Exhaustive exploration of options/approaches
Exhaustive iteration

Quality

A preference for quality
A preference to test designs
Open to feedback
Able to balance classical and expressive aesthetics
Able to balance aesthetics and usability
Obsessed with details
Rigorous

Technical skills

Good typography
Good composition/layout
Good colour choices
Good at some associated skills (e.g. prototyping, animation, imagery, writing)
Tool expertise

Wider picture

Informed by understanding of the product (e.g. research)
System thinking (how the work is connected to other things)
Product thinking (what makes a successful product)
Development thinking (how a design might be implemented)
Practical mindset when necessary
Believes in importance of accessibility

Communication

Able to articulate your design “feelings”
Able to explain design principles to others
Can clearly document/explain the design (e.g. specifications, mockups)
Good relationship-building, especially with developers
Able to “sell” a design idea