Hopefully Sal isn't holding his breath. I believe most people are hesitant to bother attempting to answer such a question as there is so much variability from person to person. (Yeah I figure he knows that and is breathing some clean Rocky Mtn air...)
There can be some generalities that will generally help the definition, but not definately.
Symmetry. Most humans identify with symmetry. Looking at a person's face, if everything lines up "normally" the person might be "good" looking. If one eye is higher than the other, and there is a scar, that person might be Igor.
Along with symmetry is "flow". Flow is harder to describe, but if you look at enough "ugly knife" threads, most examples have a lot of jagged or sharp corners. They lack flow.
Sometimes symmetry and flow can be offset by an outlier that actually enhances beauty. On the face example, a well placed mole on the cheek is often seen as a non symmetrical enhancement (Cindy Crawford for example...many pretty faces, but the mole is famous.) In knives sometimes a non-symmetrical knife with a jagged line might be beautiful...if not overdone.
Different, but not too different. Exotic inlays, some file work, and whamo, a $300 knife is suddenly a $1000 knife that any knife nut would be happy to own. Overdo the work and you might still get $1000, but it will be a one person show that makes most people's eyes water.
From a maker's viewpoint, beauty is strongly fixed with price. The higher the price the harder to make it beautiful to a large market. i.e. "Oh I love the Desert Ironwood inlay, but for a $200 knife I won't buy it unless it ..."
Sometimes what makes it beautiful just happens to override the ugly.