What separates great names in piano manufacture from the not-so-great?

Three basic things: Design, Materials, and Workmanship. Effectively combined these basics will produce an instrument of superior tone, touch and beauty. Cut corners in any area, and quality suffers.
Design: Was the piano designed to produce optimal tone, respond fluidly to the touch of the pianist, and have a pleasing appearance, or was it designed to be quickly and cheaply mass produced?
Materials: Does the manufacturer call for the best performing, most durable materials available, or the least costly alternative? The range of quality in this area alone is extensive. Natural materials such as wood and felt must be specially selected to meet the demands of the piano design.
Workmanship: Did men and women labor for months to carefully hand craft the piano, paying strict attention to detail, or was it quickly and carelessly assembled, leaving the details to be attended to later by the service technician? Quality pianos usually arrive at the dealer's showroom in relatively good tune and regulation.