From our (and every Stack Exchange site's) FAQ page on "What types of questions should I avoid asking?":
To prevent your question from being flagged and possibly removed, avoid asking subjective questions where …
- every answer is equally valid: “What’s your favorite ______?”
- your answer is provided along with the question, and you expect more answers: “I use ______ for ______, what do you use?”
Some subjective questions are allowed, but “subjective” does not mean “anything goes”. All subjective questions are expected to be constructive. What does that mean? Constructive subjective questions:
- inspire answers that explain “why” and “how”
- tend to have long, not short, answers
- have a constructive, fair, and impartial tone
- invite sharing experiences over opinions
- insist that opinion be backed up with facts and references
- are more than just mindless social fun
For more detail, read about our guidelines for great subjective questions and blog post about how real questions have answers.
I encourage you to read the full "Good Subjective, Bad Subjective" blog post linked above. The list of qualities of "constructive subjective questions" above is a summary of that post's guidelines for "great subjective questions." Note that this is not the same thing as hard rules for acceptable subjective questions, which that post says are difficult to draw.
In general, I think it's wise, with respect to any question that has subjective aspects, to apply the overall advice in the blog post:
Apply the six subjective question guidelines and see how it scores. If the score is low, close it. If the score is high, vote it up.
More particularly, this meta question concerns mainly questions that fall into two main categories: product-recommendation and how-to.
With respect to product recommendations, we've written what has emerged as a primary consideration into the product-recommendation tag wiki:
Questions in this tag should give explicit, specific guidelines describing the sought after qualities in the product. "A really good Fizzboop" is not sufficiently explicit and should be closed as primarily opinion based.
I'd say that a similar standard ought to apply to how-to questions: It should be clear from the question what criteria would make one answer better than another. In many cases, simply stating a particular problem to solve clearly is probably sufficient, since the implied overall criterion is how well answers solve that problem.