Discussion boards which are "threaded" by subject lines are difficult to follow, especially for new users. Subject lines are often inadvertently, or deliberately, changed, so that related postings will have no obvious spatial relationship, and sometimes no obvious semantic one, either. Further, the possibility of identifying what your main point is in the subject line (a useful tool, especially if the subject line is all you have) is lost. A list of a dozen messages all with the same subject line only tells you those people posted, not what their point may have been, and thus offers no help deciding what to read or keeping the course of the discussion clear.