When I assign papers, I have a general rubric that I usually grade to, as specified below. Plus and minus grades are applied to reflect finer distinctions; the letter is then converted to a number according to the grading scale (as generously as possible, so an A- counts as a 90%, for instance).
|C||A C paper has a discernible thesis (where appropriate) or topic. The author made some progress towards addressing the thesis or topic. The language used is basically coherent.|
|B||A B paper's thesis (where appropriate) or topic is clear. To support it, the author has done more than recap and paraphrase other work or make ``obvious'' statements. There is a clear flow to the text, no significant errors of fact, and no serious problems with grammar, punctuation, or spelling.|
|A||An A paper has a strong conclusion or makes an interesting point, and presents insightful analysis. The author shows an understanding of the material from multiple angles and demonstrates a synthesis of ideas from multiple sources. There are essentially no factual errors or errors of grammar, punctuation, or spelling.|
|D||In a D paper, the thesis (if present) is poorly supported or the topic wanders. ``Arguments'' are handwavy and have little real substance. Whole paragraphs may be restatements of arguments from other sources. The author may use technical language incorrectly and/or make significant errors of fact; and the writing itself (grammar, spelling, etc) may be distractingly difficult to follow.|
|F||An F paper typically has no clear topic (or one inappropriate for the assignment), its text is not especially coherent, and there are often major language issues throughout.|
The order of this rubric is not an accident. Rather than ``starting at A'' and chipping away points for errors, you should think of the sort of basically-competent blah paper you can pound out at 2am as a C, which you can edit and improve to get a better grade.
In most cases, the letter grade is driven primarily by the topic/thesis/argument/conclusion part of the rubric; if the more mechanical areas (flow, grammar, spelling) are better or worse than the letter might indicate, that's where a plus or minus gets assigned. On papers that permit revisions, a B paper (for instance) where you only adjust for grammar or spelling or other compositional issues might become a B+ paper but won't become an A- or A unless changes are made to upgrade the quality of argument or analysis.
There are six areas the Math/CS department is currently looking to focus on in student writing. These are: clearly stating a purpose; accomplishing that purpose; adequately developing the topic; logically sequencing the ideas; using appropriate language; and using proper grammar and punctuation. My comments will reflect this, and you can use this list to improve your own writing.