Also, if it's a negative behavior that has caused enough grief for the person to bother posting about it on an internet forum, you can count on some emotions of frustration to be reflected in that post. Many would call that in itself, a rant.
It would be very difficult for someone who has reached this point to take a step backward, view the situation from an observer point-of-view and to frame it in the form of an inquisitive, emotionless query.
Re this above section of your post, I think that's the key. Of course, you can't switch emotions off, but from an etiquette standpoint it's how you express them. For example, in real life if somebody makes you angry, you shouldn't yell and swear at them, even if that's how you feel. But there are polite ways of conveying how angry you are about a situation. Similarly, in posts we look for polite ways to communicate our feelings.
I find that shifting the focus to the etiquette issue helps me to deal with a situation in a polite way - because I look at it from other people's perspectives, not just the way I'm feeling. The purpose of the forum, to my understanding, is to help all of us improve the way we behave and learn more about etiquette. So it's worth considering whether a post will advance the dialogue or not. If the sole purpose of your post is just to let off steam or complain, then it's probably just a rant. But if you have an etiquette question or genuinely want to understand how to deal with a situation, then you should be fine.
I understand how you feel though, because I still struggle with how to word my posts properly to convey situations accurately and in a way that won't be misunderstood. eHell has actually assisted my written communication skills because of that!