There is a question on the possibility of a cognitive advantage to dreaming, and whilst I was reviewing the question, before I could type and post my comment on it, the post was put on hold based on the idea that it was primarily opinion based.
As the hold reason says
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.
I thought this was a good and valid question for CogSci. Another point is that the question had 10 up-votes, and a quick search on the net revealed articles such as the Harvard Health Publication Learning while you sleep: Dream or Reality stating that
A 2010 Harvard study suggested that dreaming may reactivate and reorganize recently learned material, improving memory and boosting performance.
I thought I had worked out what consitutes an opinion-based question and now I am confused. I have since been trying to work out why it was thought this question would primarily be opinion based and so I thought I would ask here what the consensus is on what makes a question primarily opinion-based?