Also see
http://www.pcgs.com/photograde/#/Lincoln/GradesGuidelines for Eye Appeal
Eye Appeal is one component of grade. For coins grading above MS/PR 60, eye appeal is one of the four components of grade.
For Mint State and Proof coins, the three factors comprising a coin's "technical grade" are:
Number and severity of marks and abrasions
Luster, or Reflectivity for Proofs
Strike, which is rarely a problem for proofs and strike is expected to be sharp, a weak strike being a deduction in the case of proofs.
For Circulated coins, there are:
The amount of wear. This is by far the most important factor in the grading of circulated coins.
Marks and abrasions. Depending on the grade, a certain amount of marks and abrasions are expected with circulated coins. Severe or unusually serious marks "for the grade" can be a negative. The higher the circulated grade, the less severe marks can be before they would effect grade.
Luster. AU (Almost Uncirculated) coins should have some original luster. For lower grades color and originality have the same effect on grading as luster does for higher grades.
The "technical" grade of the coin is the grade of the coin based on the factors above without taking eye appeal into consideration. Eye appeal either adds or subtracts from the "technical" grade, or is neutral as a factor in determining the final grade. For toning, PCGS uses seven levels of eye appeal, from "Amazing" to "Ugly". For luster on mint state coins and depth of reflectivity on proofs, PCGS uses six levels of eye appeal, from "Amazing" to "Negative".
The following are the minimum standards for eye appeal on high grades: MS/PR68 – Must have positive eye appeal MS/PR67 – Must have above average eye appeal MS/PR66 – Cannot have below average eye appeal MS/PR65 – Cannot have negative eye appeal. AMS/PR65 coin can have below average luster or color (toning) if it is outstanding in every other way.