This was initially written as a question. But since it would largely be a duplicate of this one, which is now 2 years old, I prefer to include it as an answer, shedding some light on what this is leadin us.
Raphael's answer, fairly highly upvoted, suggested a policy, fairly tolerant. I do not have the feeling that that is what we are currently doing. I am discussing it around the example of a very recent question, but I have been bothered by it for some time, while reviewing questions.
Policy against questions containing an answer is getting excessive, and we should be less systematic and consider foremost whether the gestion may be of interest to users, remembering that users are not necessarily experts. And neither are all posters of questions.
There is a clear inconsistency in the policy regarding question:
on the one hand we tell them they are expected to have researched the problem, and to show that they have;
on the other hand, if they went so far as having a solution that they are not sure of, they get blamed for asking a yes-no question.
Staying in between is a difficult exercise, and I think we are overdoing it, and we should be more tolerant, both with remarks in comment and downvoting
What motivated this remark is a recent question Categorizing P and NP problems, (now deleted, so that it requires enough rep to be read). It is 4 related questions (really 4 sides of the same coin, if I may say) aiming at testing understanding of polynomial reduction and equivalence.
This also questions the "only one question per question" policy, which should not be taken too litteraly. In this case, it is really one question, with 4 facets. I am saying that for completeness, but I do not want to be accused of raising an independent problems in an answer.
It may be more interesting to me as I have little experience in this view of complexity, though I do realize it is elementary for many CS users. But I think my less expert view is an advantage, since the site is not supposed to help only experts.
As it is, I found the question interesting (I did not check whether it was a duplicate). It made me think about these issues, and get a better understanding of what is implied by polynomial equivalence.
Having these questions, less obvious to me than I first thought, and the answers of the poster, together with his reasonning (which may call for more than yes/no answers) was an opportunity to discuss the issues, because some answers are wrong (I think) and because some are true, but maybe not for the reason given.
Fearing the question might get closed (from 2 downvotes, from 1 closing vote, and from the first comment), I edited the question to attempt to give it more conformance to question standards. I might also have risked an answer, I do not know. Unfortunately it seems that the users got the first messages (he has 10.2k rep on Stack Overflow, so he knows the rules of the game) and deleted the question himself, which I regret.
This question is only an example, and the opportunity to voice more precisely issues that have been bugging me. And at the same time, it may reopen the issue and raise awareness of the policy.