Each professor uses a distinctive set of grading criteria. This is because grades reflect what a professor values and emphasizes, and what gets valued and emphasized varies from discipline to discipline, from course to course, from professor to professor--a healthy situation in a liberal arts college.
Grades are not the end-all of a college education. They are significant, however, inasmuch as they measure how well you have satisfied a professor's expectations, expectations that presumably you share. A good rule of thumb, especially given the diversity of expectation in the college, is to understand as clearly as possible what the criteria for judgment are in each of your classes.
Just to give you an idea of what professors value in writing, here is a representative description of an A, or "Superior" paper:
Perhaps the principal characteristic of the A paper is its rich content. Some people describe that content as "meaty," others as "dense," still others as "packed." Whatever, the information delivered is such that one feels significantly taught by the author, sentence after sentence, paragraph after paragraph. The material is not predictable; perhaps a key word for A writing is "surprise." The A paper dares something.
The A paper is also marked by stylistic finesse: the title and opening paragraph are engaging; the transitions are artful; the phrasing is tight, fresh, and highly specific; the sentence structure is varied; the tone enhances the paper's purposes.
Finally, the A paper, because of its careful organization and development, imparts a feeling of wholeness and unusual clarity. Not surprisingly, then, it leaves the reader feeling bright, thoroughly satisfied, and eager to reread the piece.
To illustrate more concretely how writing grades are sometimes defined, here is the English department's agreed-upon descriptions of grades from A through F. Note how they are calibrated to the five areas of writing named in the grid above.