I've taken what follows from one of my former professors, Dr. Karen Coats. This is the rubric she used for all of her courses including undergraduate and graduate courses. She articulates how I evaluate papers as well. I've pretty much copied her standards verbatim.

As Karen writes:

Let's start with what is average. Average means that you are not at one end of the continuum, or the other. Remember high school? The people on the one side of the continuum, academically speaking, took honors or merit classes, got As and Bs in them, and applied to college. The people at the other side made other choices.

You're a college student. That means you have left a large segment of the population of your high school, your hometown, your world to their other choices, and this changed the nature of the continuum. In other words, instead of being compared to the whole spectrum of people who make a whole spectrum of choices, you are now in a self-selected, teacher-validated group that have made the choice to pursue academic glory and higher learning. Grading scales reflect that change by becoming "tougher." This is because everyone in the pool is used to As and Bs, making the kind of work and thinking that used to get As and Bs now seem average.

That said, these are the standards for a C:

If your work is clear, concise yet thorough, and competently executed, this counts as a C.
Conception: Your idea for your paper should reflect that you have read, thought about, and paid attention to the way we have talked in class about similar issues. Your main point should be clearly stated and defended with appropriate evidence. You should remain focused on your topic throughout your paper, and you should have thoroughly examined the aspects of your topic from your perspective. Your ideas should be internally consistent.

Organization: Your paper should have a logical, clearly identifiable organization. Each paragraph should address only one aspect of your topic, and when you change aspects, you start a new paragraph. Transitions between paragraphs should be competently handled. Your strategy, that is, how you manage the interweaving of your idea and your organization, should be standard and straightforward. For instance, if you follow a traditional pattern of an introduction that includes a flagged thesis statement ("in this paper I will..."), then proceed with evidence and close with a restatement of the initial problem, then that's a standard, straightforward organization--a C strategy.

Style: Your style should be clear and readable. You should sound intelligent.

Grammar and Mechanics: Your paper should not contain distracting errors in grammar or mechanics. Minimally, you should have run a spell-check program, and you should know the difference between a complete sentence, a fragment, and a run-on.

To get an A, then:

Conception: Your idea should contain some new, perhaps surprising, element, some angle that is uncommonly thoughtful and insightful. You are not rehearsing other people's ideas, and you are going beyond the kind of reading that an average intelligent person might do. You expose and challenge the explicit and implicit assumptions of the text. If you are incorporating research, you will have WORKED your sources--using what supports your argument, and acknowledging and dealing with what challenges it.

Organization: Your organization should be flawless. You should address and work through opposition to your argument early, and spend the rest of your time building a strong case, supported with evidence, that moves from weaker points to stronger ones. If you are incorporating research, you will spend some time positioning your argument in the context of the larger conversation.

Style: Your presentation should be artful. You have obviously paid attention to the way your language sounds as well as what it says. You have found a way to make your presentation style match the content of your paper (other than a groovy font style!), perhaps through a sustained metaphor, or a particularly apt example that you carry through and refer to in the entire paper.

Grammar and Mechanics: Your paper should be absolutely clean and free of grammatical and mechanical errors (or very close to it).

Do you need the rest? Okay.

B:

Conception: Your idea will be better than average, but you may have overlooked or not acknowledged or interrogated the assumptions that inform it.

Organization: Your organization will be strong, but the signaling might still be a bit clunky--you may find yourself using a lot of directional phrases because your argument doesn't flow naturally. (Ex. "As I said earlier..." "Firstly, secondly, thirdly...")

Style: It's clean, readable, there's a consistent sense of voice, and there aren't any places where a reader has to go back and reread a sentence just to understand its structure.

Grammar/Mechanics: Very few (almost no) errors.

D:

Conception: Your idea will be immediately obvious to a casual reader--a no-brainer. Remember, this is college, not common sense (c:. As I stated above, as a college student, you start out above average in your approach to reading and writing, so reverting back to the obvious puts you below average in reference to your peers.

Organization: Perhaps you split your focus (which means you start out talking about one thing and shift to another). Or you jump from one idea to the next with no logical strategy or transitions. If there is no plan, or if you don't stick to the plan, this is faulty organization.

Style: Reads like a casual chat with friends, rather than a smart, academic paper.

Grammar/Mechanics: Consistent problems in sentence structure, no visible attempt at proofreading.

F:

(The most common cause of an F is a failure to adequately address the assignment. For instance, if I specify that this assignment is to be researched, or if it is to address a certain topic in a certain way, you have to at least complete the assignment.)

Conception: No clear idea governs the words on the page.

Organization: No plan is evident, much less achieved.

Style: Reads like a casual chat with friends.

Grammar/Mechanics: Consistent problems in sentence structure, no visible attempt at proofreading.

The above quoted from: http://lilt.ilstu.edu/kscoat2/grading.htm