When marking your papers, I will use the notations described below when making editorial comments. Of course, if your paper is perfect you won't find any such notes!

awk	Awkward wording - the meaning of the sentence is difficult to discern.
ROS	Run on sentence - at least one too many phrases in the sentence.
ref.	A reference to the literature is appropriate for this point.
?	The point you are trying to make is not clear.
^	Insert the word or phrase above at this point.
//	Start a new paragraph here - too much contained in this paragraph.
reword	Although the meaning of the sentence is discernable, rewording would make the point even clearer
NAS	Not a sentence - most likely the subject, object, or verb is missing.
#	Number the pages please.
	Good point!
	Great point - likely because you have made a link to one of the main themes of the course!
ref. list	You need a reference list or bibliography - even if there is only one reference
P?	Purpose unclear - What is the purpose of your paper? What is your main argument in this paper? Your purpose should be CLEARLY stated in the introductory paragraph. This might also indicate a spot where the point you are making is not consistent with the purpose of your paper.
	The word is misspelled,for example: use the possessive of it - its - properly; contractions like "isn't" - should NOT be used in a formal essay; use the Canadian spelling of words like favour.
Q S	Quote sentence - A direct quotation does not stand on its own. (Only rarely is this acceptable - e.g., a pithy quotation prior to the introduction of a paper - for reference I prefer not to see these unless you directly refer to them in the introduction). A quotation should be prefaced by an introductory phrase. For example, Baxter claims that, "This quotation was prefaced by the phrase for example" (author year, page number). Note that the citation for a direct quote must include the page number.
great	Better word choice required - if a word is crossed out and perhaps, another suggested, it may just be a suggestion that the original word may not portray the meaning you seem to intend. In other cases it may be a grammatical error.

Thanks to Jamie W. Baxter for providing this list of commonly used notations.