I completely agree that there is much too much room for interpretation. I would prefer there be hard and fast rules, but as long as "sexy" is considered acceptable criteria for images to be posted here, we will have problems. One person's sexy is another person's boring. As long as we allow such subjective criteria, we will have issues. And I'm OK with that, since, as I have already said, I am going to continue to Mod the way I think I should, which will mean jihading posts other mods have approved.
Some of the criteria I use includes the amount of appropriate showing, which would include cleavage. I put obvious pokies or c-thru to breasts in that category. I DON'T put c-thru to bra in that category, at least not all the time. And I don't care how sultry and made up a celebrity is, if they are fully clothed or it is only a very attractive head shot, it does not belong. Two-piece swim suits pretty much always fit here. Ditto most underwear/lingerie shots. But regular glamour shots, or candid shots (including red carpets) must show some skin either in cleavage or a VERY high cut leg.
That's largely how I approach my work here.