Step 1: The question is about the defining characteristic of the boots made by Mr. Gessler in the story "Quality" by John Galsworthy.
Step 2: The story emphasizes Mr. Gessler's extraordinary craftsmanship and dedication to his art of shoemaking.
Step 3: The narrator repeatedly highlights that the boots Mr. Gessler made were perfect in every way, especially their fit. He says they were mysterious and wonderful, and they "never failed to fit." This perfect fit was the hallmark of his quality, which stood in contrast to the mass-produced shoes of big firms. While they did impress and satisfy, the most specific and praised quality was their perfect fit.