The paper should reflect the theme of the course.
You should be sensitive to the point of view the professor is trying to present and to the scope of the course. A good paper should reflect the theme of the course in some way, even if you do not agree with the professor's approach.5 Consult your TA or professor before you invest a lot of time and energy investigating a topic that might not be appropriate.
For example, when you are writing for a class that focuses on some aspect of cultural symbolism, and you find yourself discussing astrology, King Tut, and holistic hang gliding, then you're stretching the boundaries of the course. You will probably find that you are stretching the boundaries of your GPA too.
More realistically, if your professor has been talking for weeks about political conflict, then a paper in which you marvel at the harmony and smooth integration of culture--and by implication deny the reality or significance of conflict--will probably raise some eyebrows, but not your grade. But a paper about the problems of political leadership in Arab villages in territory occupied by Israel, or about lineage feuding in classical China, would more appropriately reflect the theme of the course.