I agree tone is important. And, that's something that can be hard for a person to judge of what they themselves have written, unfortunately. If something is gossipy, judgmental, opinionated, that usually feels more like a rant. Maybe imagine who you're "speaking" to--with your BFF you might try to be funny and colorful, to entertain her; but I think here, a person is taken more seriously if they adopt a tone of "just the facts," like they might say to the police or in court. If someone is struggling, perhaps they could let a friend read their first draft, or send it via PM to a friendly forum member, to get a second opinion?
Another thing I've noticed is that sometimes a person will go on at some length (a paragraph or two) about X, and then say, "I know X isn't rude, it just bugs me, my real question is about Y." Well, that's going to be noticed--inevitably at least one person will say, "X is not rude and talking about it like that makes you sound very judgmental." And you might say, yeah I know, I specifically said it isn't rude--but including the section about X colors the rest of the post for people, distracts them, and perhaps makes them distrust the OP's take on other things. So after writing out a post, especially a long one, the OP might go back over and edit it, considering whether each part really is necessary and gets to their point.
I do think it can be tough trying to decide what background is necessary. No opening post is going to be perfect! And not even the longest post is going to describe every nuance of a relationship. I usually write a lot, because writing it out can be therapeutic on its own, then go through and slash a lot of it (sometimes even deciding not to post at all).