Well you'd need to look at what makes an Abbott's an Abbott's. That would be bright red and orange coloration and thick black borders. Therefore a "holdback" would have exceptionally bright coloration and/or thicker than the average (for an Abbott's) black borders.
Rarity is one "prize winning" trait. If it's the only one (color morph, pattern, etc.) people will like it, because those that have been breeding snakes will appreciate its uniqueness - that something new has been created via selective breeding.
Color is another "prize winning" characteristic. Breeders shoot for jet black rather than dark brown, bright white rather than off-white, yellow rather than cream. One of the goals of selective breeding is the refinement of color. The more "extreme" the color, the better. Solid color is "better" than a mottled mix of color.
In addition to this, there's pattern. Uniformity is key here. A clean, evenly striped corn will find more fans that unevenly striped or broken-striped examples. The same could be said for zigzag and banded corns.
These are three keys that breeders strive to produce to "one up" each other: rarity, color and pattern.
CSG