Sites on SE understandably vary in their tolerance for opinion based questions (OBQ).
Although I am not a member of the Physics SE, I imagine it is very intolerant of OBQs unless they are about the frontiers of physics. (Possible example: Will the LHC discover a particle not predicted by the Standard Model?) English Language and Usage is also intolerant of purely OBQs, although because of the many dialects of English, what seems like an OBQ can elicit a scholarly answer about a rare and obsolescent usage. Interpersonal Skills has Qs with a high coefficient of Opinion Basicity. The Q of @Charlie Brumbaugh, Is it poor etiquette to ask fellow backpackers where they have been/where they are going?, would not raise an eyebrow on IPS, although it doesn't belong there because most of its members have shown no signs of being outdoorsy types.
In my opinion, the VtC on Charlie's Q was misguided. The Q is about how to avoid frightening or irritating or otherwise provoking a member of, or a small pack of, Homo sapiens encountered in the wild. Homo sapiens is a species with aggressive and predatory instincts that are well, but imperfectly, controlled by cooperative instincts and by social conditioning. It is uncool to frighten a member of this species, and sometimes dangerous. This site welcomes Qs asking how to avoid enticing, surprising or frightening Ursus americanus (black bear), who is roughly the same size as Homo sapiens. The dangers of provoking Homo sapiens, or encountering a predatory Homo sapiens beyond a trailhead, are roughly equivalent (perhaps to an order of magnitude or so ?) to provoking Ursus americanus, so this Q is well within the scope of TGO.
Also, note that an anthropologist could write an authoritative technical answer on how Homo has developed facial and body signals and greeting rituals to indicate friendly intent.
So to my Q: What makes a Q on this site an OBQ? Examples of Qs closed as OBQ on this site would be helpful.