Grades that professors derive form a non-quantitative evaluation sometimes confuse students who have not written a great many essays before. So below I've outlined not an exhaustive criterion, but rather a profile of several typical qualities that earn particular grades. I hope this is helpful, but please see me if it raises any new questions.
A Range (A-, A, A+)
An original and lively thesis that is at least partially successful. Thesis is argumentative and controversial, sensitive to the text and its contexts, working against a strong counterargument into the heart of the text and enriches the reader's appreciation of its artistic significance. Inventive or even simply sound choice and use of secondary materials. Strong evidence of having considered and re-considered both text and paper drafts. Plentiful and to-the-point close readings (but no lengthy quotations). Lively and grammatically correct prose. Strong sense of paragraphing, structure, logic and audience. Proper manuscript format, including notes and works cited list.
B Range (B-, B, B+)
Original thesis that is partially successful, but is either underambitious or logically flawed. Contention with challenging counterargument. Sound use of secondary materials. Evidence of familiarity with nuances of the text. Evidence of multiple revision. Standard use of English (no more than, say, three errors per page). Some insightful close readings. Logical paragraphing. Good manuscript format.
C Range (C-, C, C+)
Interpretive thesis, but one that is obvious, superficial, or not especially relevant to the core of the text's artistic meaning. Contention with counterargument unsuccessful or absent, or contention with weak counterargument. Minimal but relevant use of secondary sources. Mostly standard English (three to seven errors per page). Some references to text (not to plot). Inclusion of mostly proper notes and work cites list. Some sense of methodology, paragraphing, Evidence of some revision.
D Range (D-, D, D+)
Thesis that is descriptive, illogical, provable (factual), obvious, or insignificant. No counterargument noted or implied. Minimal and/or irrelevant use of secondary sources. Substandard use of language (typos or errors exceeding seven per page consistently). No notes. Improper works cited list. No clear sense of methodology. No evidence of having sufficiently familiarized oneself with the text. Little evidence of revision. Improper manuscript format.
Papers that lack thesis, that are grammatically unreadable, that are not research papers, or that lack evidence of familiarity with the text.