(tl;dr version: "Primarily opinion-based" is designed to stop "What's the best X?"-type questions. It's not supposed to be used on every single question that involves opinions.)
Until last year, one of the reasons that questions could be closed was because they were "not constructive":
As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
In 2013, the question closing process was revamped across the SE network, in an effort to make it appear less hostile and to more effectively help users improve the quality of their questions. Several existing close reasons were retired, and new ones were introduced in their place. The new option closest to "not constructive" is "primarily opinion-based":
Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.
While this change was entirely well intentioned—after all, nobody likes to be told they're not being constructive, and it hardly helps them improve—an unintended consequence of the change is that "primarily opinion-based" is being used at ELU to close questions it was never designed to close. To understand why, we need to look at why we had a "not constructive" close reason to begin with, as well as our own guidance about what constitutes a constructive question.
The archetypal "not constructive"/"primarily opinion-based" question is "What is the best [car, game, pet, movie, dictionary, study method, etc.]?" There is no right answer, and it can only lead to rep-pimping and mental masturbation, as everyone and their dog stops by to leave their two cents and shoot the breeze. It was widely apparent fairly early on in the history of SE that these questions have no place on the network, and it's easy to see why. Questions like this one are cute, and they can be very popular, but if they start to absorb all the energy on the site, real questions suffer. (Incidentally, this is a big part of the reason the "community wiki" option exists: it provides a mechanism to stop users from gaining thousands of undeserved rep points for lazily pasting their favorite cartoon into an answer.) Our FAQ provides an excellent list of the kinds of questions that are not constructive:
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?”
- there is no actual problem to be solved: “I’m curious if other people feel like I do.”
- you are asking an open-ended, hypothetical question: “What if ______ happened?”
- your question is just a rant in disguise: “______ sucks, am I right?”
Unfortunately, many people at this site think "primarily opinion-based" should be used to shut down discussion of any and all topics having to do with opinions about English. To select the most convenient example: this question, an intriguing and well-written query about the reasoning behind linguistic prescriptivism, currently sits with 4 close votes, and may in fact be placed on hold before I finish this meta question. All four voters in favor of closure have chosen the "primarily opinion-based" option. But why? The user isn't asking whether prescriptivism is better than descriptivision. He clearly doesn't care for prescriptivism, but he's not just saying "Prescriptivism sucks, am I right?" It's a legitimate question about the philosophical underpinnings of prescriptivism. It should be entirely possible to formulate a comprehensive and useful answer that invokes Robert Lowth and Strunk & White and explores the role of Greek and Latin antiquity in forming the opinions and assumptions of the learned class in Britain and America in the eighteenth and nineteenth centuries and so forth. Instead, people see the title of the question—"What is the point of prescriptive grammar?"—and the "primarily opinion-based" light bulb lights up in their brains, and the close votes pile up. And that's not right. Just because a question asks about opinions does not mean it is asking you for your opinion.
It should be obvious to everyone here that a great deal of English usage boils down to subjectivity and opinion. Register, formality, and etiquette are vital components of usage, yet talking about them unavoidably means talking about why one word or phrase is considered "better" than another in certain contexts in the greater society, or in or between various subsets of it. Linguists have spilled barrels of ink exploring exactly these questions in classrooms and in scholarly articles. To suggest that they should not be asked here is insanity.
According to our FAQ, 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
Before voting to close a question as "primarily opinion-based," ask yourself whether it meets these criteria. If it does, leave it be. Try assuming that when the questioner asks something like "When should I say X instead of Y?", they're inquiring about cultural norms, not just bending your ear about what you, personally, think about it. If necessary, edit the question to clarify it. A word or two in the right place can save a question from the garbage and turn it into something that enlightens everyone.
Disclaimer: The aforegoing is all just, like, my opinion, man.