It's fair if there's a rubric that tells you specifically what's required to get an A and you just didn't do it. I think a lot of people in this thread are implying that it's alright for teachers to totally wreck your GPA because of their own personal opinions about what "average" ought to be, or about what "C" ought to mean. They should still try to be at least somewhat objective. If they don't explain what they're looking for, they have a bit too much freedom to destroy you for whatever reason.
My English professor this semester is known to be tough--and even hated for it, to some extent--but, based on what I've seen so far, I respect her because her criticisms are very detailed and specific. If she had written on my last in-class essay, "Meh, you get a C," I don't think that would've been alright.
In contrast, the guidelines my Sociology professor provided in his syllabus were totally inconsistent with what he was looking for, apparently, and that annoyed me a little. He specifically told us not to summarize the articles we were responding to, and, the next class, he brought us an example of what an "A" was: an elaborate summary in which the writer simply agreed with all the article's main points, without necessarily building on it or anything. But anyway, I did my next assignment more like that, and all was well.
tl;dr educators should help you