I have been a member of many online communities over the years. Some for work, others for fun. I do often wonder what specifically is the magic to making some sites work and others not. I think the biggest problem that is gripping the forums here is the fact that often people get so involved in correcting one another and nit-picking details that they forget to keep the original poster in heart.
Of course the biggest problem with doing this is the overall experience that from the poster is that they feel like their question is answered but then dwindles off into confusion. Which may even discourage them from reading a well-written answer that was given to them prior to the thread's digression. Though it is not always clear to the original poster where the digression began. Thus their questions remain ultimately unanswered in a way that is satisfactory to them.
That is my two cents.