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It's going to vary depending on master's or PhD, subfield, etc.
That said, "near-publishable quality" shouldn't be taken to literally mean that you tried to get it published. Rather, the paper should be research oriented, addressing a substantial philosophical argument or a paper that shows your ability to entice out nuances in extremely difficult material. Faculty don't want to read your book report or your glorified timeline of 20th C. French existentialism. The former works are quality that editors love to put in their journal. The latter faculty make themselves read after their fifth whisky.
I think Brown's guidelines for prospective philosophy applicants is the best there is.

A few brief responses:
--No, it needn't be near-publishable, because that's an unattainable standard, as you note
--Philosophical creativity is a hugely important feature of a good sample, since it demonstrates philosophical aptitude transcending the sample's topic
--A good sample engages with scholarly literature (preferably recent), showing grasp of relevant literature but spotlighting the applicant's own arguments, which should address a problem, lacuna, etc. in the literature
--A good sample shows why non-specialists and specialists alike might care about the topic, esp. via illuminating writing style