I agree that the Slate-Magnum galleries are often quite good, and I often enjoy them for the opportunity they offer to see these pictures.
But I would also assert that if this forum had a paid staff to curate and market what is in the galleries, the body of work would be comparable.
I have spent many, many nights seeing local and national music groups in my town, and if anyone thinks nationally known music acts are the cream of the crop, they are sadly misled. Quite often the difference between fame and anonymity is motivation and opportunity, not skill. I've seen enormously talented groups with a fresh and invigorating approach vanish for no apparent reason other than people's lives (jobs, families, etc.) got in the way of entertaining others. Can't say I blame them.