The alt-text of the downvote button gives 3 reason why a question should be downvoted:
- The question does not show any research effort.
- The question is unclear.
- The question is not useful.
The first two are pretty self-explanatory IMHO. If a question can easily be answered with one query on Google, then it doesn't show any research effort. If anyone asks a question with vague wording or ambiguous terms, then it's unclear what they're asking. The third isn't really straightforward to me.
I usually upvote a question which piqued my interest, thereby signaling I'd like to see someone post an answer. In that case the question is useful to me in some sense (if only to quench my curiosity). The reverse doesn't really work, i.e. if I don't find it interesting I ignore the question because I don't find it useful. I wouldn't downvote it merely because the topic doesn't have any utility from my perspective.
So in the context of Skeptics, when should a question be deemed "not useful" (i.e. to the point where it makes sense to downvote)?