But I would also argue that 4-5 levels of quality is too much, because is the grader comparing papers to other papers, or to some ideal paper that exists in his mind?
I think, in general, the grades A, B, and C map to the following standards of acceptable work:
A: That of a student intending graduate study in the field. B: That at a major. C: That of an ordinary student, e.g. a non-major.
Hence, C is failure in graduate school, whereas a C in Calc 2 for an English major is unambiguously not failure.
F obviously represents failure. D is essentially a high-fail that's counted as a pass. It means, "you didn't pass this class, and you shouldn't use it as a prerequisite, but you didn't do so poorly that I want to set you back a course." D would probably vanish if college weren't so fucking expensive.