I think one of the defining characteristics that differentiates "good" from "great" is discipline. Good students possess great skills, but they lack discipline.
Uh, good students (if you mean the ones that get good grades) have incredible discipline. Indeed, given a base amount of talent, discipline is all that is required.
I was just commenting on the quotation in the post.
I think it would be erroneous to conclude that a student is not "good" because he/she receives a "C" in an economics course. Perhaps the student was very ill that quarter or was busy building some really great thing.
But even if they slacked off, they may want to be disciplined. Better yet, they may be willing to listen. In which case, one can probably be taught discipline.