Alright, a bit late to the party, but I do have something to add to this. Probably nothing that hasn't already been said (didn't read past the OP, lololol) but I think the nature of my position on this forum changes things up a bit. As most of you probably know, I was an administrator until very recently.
The Borderline NSFW warning has irked me for a long time now, to be honest. We have had, I'll admit, a glaring lack of consistency in the definition of this rule and methods of enforcement. Should have probably spoken up sooner about that. However, when somebody did file a dispute over receiving such a warning, I did do my best to render a fair verdict and overturned a few of them, so we're by no means apathetic or plugging our ears to any outcry arising over this issue.
As has been noted by EP, many times have moderators made what appeared to be very strange distinctions as to what qualifies as an infraction, sometimes appearing contradictory. I can assure you that there was never any nefarious motives or the like, but the "borderline" warning is ambiguous by nature. On the positive side, it keeps people from getting too heavy-handed a reprimand for an infraction while conversely causing some others to get reprimanded for something that was wholly undeserving of punitive action. The problem is that there is no solid definition for what constitutes "borderline," and it resists further elucidation despite sincere efforts by staff members to establish some guidelines. That said, if the staff can't even form a consensus and know exactly what they're doing in regards to this policy, how can they expect those not on staff to adhere to the rule? They can't.
My preference would be to annihilate this warning reason altogether. Prior to my resignation yesterday, we discussed this matter in an admin call on Skype, in which I voiced my support for abolishing this rule and being a little more flexible with the standard NSFW warning except in extreme, obvious cases such as pornography and gore.
Let me reiterate that the moderation and administration have been doing their best to be even-handed here, but this situation too easily lends itself to individual discretion and circular discussions among themselves. They're doing the best they can with what they have, so perhaps it's simply time to restructure the system a little bit to avoid further misunderstandings and make everyone's lives a little easier.