SFF.SE's downvote arrow lists 3 criteria for when a question should be downvoted ("No research", "unclear", "not useful").
The first 2 are reasonably clear-cut, even though the "unclear" one has potential to be quite subjective.
But it seems that we as a site have a major deficiency in a well-understood and agreed upon set of criteria of which questions are "useful" vs "not useful".
Given examples below, I feel that we should have a very well developed and settled set of guidelines of what the site community as a whole views as "useful" and especially "not useful".
If we do, we CAN use it to explain to newer voters what they should consider when making voting decisions (we can't - and shouldn't - use it to police votes, however). I have personally had many cases where I talked someone out of a DV or VTC by explaining Meta guidelines that they weren't aware of.
Examples would be:
Some people once in a while pop up who openly declare "Plot hole explanation" questions as not useful.
Yet, as the Meta consensus shows, the community settled on such questions as being perfectly fine and useful.
Some people (especially in chat, too lazy to search meta) declare any question asking about "speculative" situation as not useful ("Can you cast Avada Kedavra while doing triple somersault while dressed as a clown?" - responded to with "Not useful, since that situation never occurred in canon").
Some people consider any questions clearly answered by books to be not useful.
Please provide ONE guideline per answer; either a positive (X is not useful; + Reasons) or negative (Y is NOT 'not useful' + Reasons). This way people can vote agreement or disagreement with each guideline.