In the comments for JoeTaxpayer's answer to this question about the "appropriateness" of assigning extra credit homework, some users have begun to discuss down-votes that seem to be based upon opinions presented in the answers, not upon the quality of the answer.
On Christianity.SE, with questions of a religious nature, you can imagine that this might be a significant problem; however, it's not (anymore). On that exchange, questions that seek subjective truths are considered off-topic. Suppose that that same rule was enacted here. For example the question,
Is it appropriate to assign an extra-credit homework question on the next lesson?
solicits an opinion, and so would be considered off-topic as a truth-based question. Answers to the question cannot be neutral, and must present the answerer's opinions about whether or not that is appropriate. Some users might consider answers that are not in accord with their own strongly held opinions to be "bad" answers, and down-vote them. Answers with many votes appear to visitors to be the de facto (community-) accepted answer. On the other hand, the question,
What are some arguments for and against assigning an extra-credit homework question about the next lesson?
does not solicit an opinion supported by arguments, but asks for arguments for and against doing that. Since a complete answer would be neutral, this question is on-topic. Partial answers, even if presenting only arguments for or against, are doing exactly that and need not be interpreted as opinions. It is more likely that dissenting voters, for this question, will up-vote strong arguments either way, and silly or invalid arguments will find their way to the bottom of the stack. The most complete and thoughtful answer will rise to the top, which in this case would include arguments on both sides of the fence. In the example question, JoeTaxpayer's answer contains an excellent argument against the practice in question, but since the question solicits opinions, his answer comes off simply as a strongly supported opinion, and the popularity voting is underway. I would really like to see that argument included in an answer to the the "on-topic" question I gave above.
Note: New users on Christianity.SE often become (very) frustrated when their first few questions are closed for this reason, because they tend to ask subjective moral or theological questions. Mine were, and I was. But as I explored and learned more about the site, I understood that the site's purpose and aims were not congruent with my presumptions. Some users will not give it that second chance.