Guideline: an OpiWiki opinion requires at least one argument or a point of contribution
We decided that for some topics, such as topics related to politics, we will be requiring the opinions to contain at least one argument. Now, this isn't a strict rule. We won't be scanning the opinions to extract arguments from them. The general rule is that an OpiWiki opinion must add at least a bit to the discussion. If there are no arguments, but there's a bit that enriches the discussion (it can be something related to personal experience, some info, etc.), it's ok.
Why this rule? At OpiWiki we have some goals set. One of them is bringing the Internet discussion to a new level. We also want to grant the word "opinion" a better name. We believe that opinions like "he's the worst", "he's the best", or "he's an idiot", 1) lower the quality of our site and work against reaching our goals, and 2) don't add anything valuable to the discussion. Now, people who don't have anything more to say are not restricted from contributing their opinions, as we also have a voting system, and we believe that a vote is more meaningful in such cases.
You can learn more about the guidelines at opi.wiki/help/discussion#b.