I'll downvote in an art share community if:
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The art is politically trolling or otherwise deliberately offensive.
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An artist is spamming there with too many posts in a short time.
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The artist is self promoting too hard in the title/comments. (Or includes any sort of 'clickbait'.)
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The art is really, really, really bad. Like, "I've seen toddlers draw on the wall with crayon more skillfully than this" kind of bad.