(I wrote this and was pointed here to Meta.)
At Network Engineering we get very many questions which are off-topic, typically for being homework, repeats, or not fulfilling the non-obvious but quite strict guidelines (networks should be professionally managed, professional equipment etc). Surely many other sites have the same issue.
One important reason for this is that the "Ask Question" button is right in your face; "What to ask" is at least three clicks away. We say PLEASE ASK, without caveats, and then, often within minutes, we say BUT NOT THAT.
This is incredibly frustrating for the new people, who immediately (and sometimes volubly), form the impression it's unwelcoming. It's hard to say they're wrong.
This is not a new observation: a year ago it was considered Time To Change.
Please can we put some How-To-Ask help somewhere effective.
All I'm suggesting is link next to the "Ask Question" button, perhaps labelled "What to ask" or "Guidance". (Others have suggested something clever, but I'm just talking about basic user experience competence: most new users are doing it "wrong", that means we should explain better/clearer/ differently/elsewhere).