I think NSFW designations, when it comes to content warnings/content filters, should be expanded to include specificity. Like how on ESRB ratings in the US, there's the letter rating to give you a general idea on who it's appropriate for, but then it lists the things that resulted in that ruling (violence, strong language, nudity, sex, etc.) so you can still make an informed decision, "this says it's rated T but mostly due to language, and I don't mind if my seven-year-old hears bad words so I'll still buy it for him" vs "it's rated T, but there's nothing to say why so I don't want to risk something I don't want my kid to see"

This can be applied to artwork, too. It can be flagged as NSFW or SFW, but if flagged as NSFW it has explanation (full nudity, suggestive pose, suggestive language, partial nudity, gore, etc.). Users would be required, when marking something as NSFW, to choose from a list of reasons. Of course if someone takes porn and marks it as SFW, then it won't matter but that can happen with the current system, too.

As far as figuring out if your art is SFW or NSFW, there should be published guidelines listing what can make an artwork NSFW.

This would, obviously, be something each site would have to determine from a moderator perspective. There isn't really a good way to get *everyone* on the same page as far as classifying NSFW/SFW.