Post what comes to your mind.
I absolutely disagree. Posting "what comes to your mind" can lead to false informations (maybe you don't remember right) and, obviously, not always it is useful or helpful to other people.
in my opinion, make constructive posts very simple, remain in the thread title track and not out of topic.
does not have to be long, quite short, dense, clear and useful.
I agree with what you said. It is popular that "Long post = constructive post" but that is no true. Every post can virtually be "conscrtuctive", even one-line ones.