I'm not sure they will give much more information than what is already given. The guidelines are vague, because comics and novels are art forms and often require to be looked at in a case by case basis.
Two stories can be extremely similar, but slight changes in presentation can turn a scene from completely fine to gratuitous. I know that people are looking for clear rules like "no erect penis, no liquids, no extreme gore", as well as definitions of what all those rules include. Yet, even if you would compile a list like that, you will still have a million open questions and people showing you their scenes and ask: "But, is THIS ok?"
Overall, you will often have to use your own best judgement. If you aren't certain, use the Mature tag. If you are REALLY worried, censor.
I also don't understand why so many are hesitant to add the mature tag. It often attracts readers (I myself have a tendency to click on episodes marked M first, yes, I'm guilty), and as a creator I see it as my responsibility to ensure that my readers don't get into hot water by reading something they, or people around them, may find offensive. If I look through comics during my lunch break at work, I would be rather displeased at a random naked person popping up at my screen - and I'm someone who is perfectly comfortable with all sorts of bare skin being shown.
It isn't my choice as a creator to decide what my readers get to find offensive or not. I have to think of what others may have issues with, and work accordingly. Just because I have no issues with a torn off head, I don't get to leave off the mature rating just because I went light on the amount of blood shown.
I think Joanne's list is a great start and covers most scenes. Some unique cases will require to be up for discussion and staff's discretion.