I feel like, realistically, people probably just subconsciously rate a given main article along each of the five dimensions you listed (and possibly other ones), and then mash all the ratings together to give a single rating along a general "good/bad" spectrum (which then falls above or below their own main "cutoff"). So a formal set of criteria for main would be nice I guess, but it seems like it would be unlikely to capture exactly what people people mean by "main-ness" anyway - in most cases it's just an intuitive judgement call. I would add, though, that one of the biggest things I look for in a main post (and I don't think I'm atypical here) is insightfulness - presenting new thoughts or ways of looking at things. Which is sooort of captured by factor 2 that you listed, but not entirely.