Two years ago, Equestria Daily linked to a bunch of shipfic collections (#1, #2, #3, #4, #5, #6; no, I've not actually read any of these), and they all contain a rather useful scale for rating content in terms of sauciness:
- Fluffy — nothing serious, mostly cute
- Fuzzy — a real d'awww scene for the heart
- Snuggly — hugs, pecks on the cheek etc.
- Tender — kisses, lying down together
- Romantic — kisses with tongue, massages
- Cheeky — moaning, overtly massaging wings, hoofs etc.
- Saucy — very, VERY suggestive
- Whoa! — describing actual sex
It really sounds quite useful — not just for fanfic but also for other kinds of art, although obviously it's focussed more on shipping than on solo depictions.