User-friendliness is the inevitable result of a smart design approach, not the starting point. Robert Hoekman, Jr lists three criteria to help you develop a useful design brief that will ultimately yield a great product.
“User-friendliness is a result, not a tactic. Intuitiveness is a quality, not an approach. Neither term is tangible enough as a design objective to inspire a great product. No matter how strong your jaw, you can’t sink your teeth into vapor and expect it to taste like Apple. You’ll only hurt yourself trying.
To achieve the result of user-friendliness, the design criteria you spill out onto the proverbial page need to meet a three criteria of their own.”