So, in general terms I am not sure when something is primarily opinion based here on UX.SE as UX is not an exact science, so every question is at some level opinion based. The skill lies in approaching the question in an objective fashion trying to cover as many facets as is necessary.
To be slightly more specific I came across this question and considering the top two voted answers I would definitely not say those answers are opinion based. Sure, there are some poorer answers that mostly just express one sided and/or unsubstantiated opinion, but even considering all answers those are in a strong minority, yet none the less a moderator saw fit to finish closing the question. Additionally it was decided that a question like Why do washing machines have windows? is on topic which makes it even more confusing.