and, by extension, in a post, which is read (or not) by many more people

Agreed. But for Calli's benefit I add this clarification:

There is nothing wrong with a "long" business letter [or "long" post for that matter] if the writer considers that all of the points covered must be addressed, or should be addressed, but each of the points should be expressed succinctly, that is, clearly and concisely, and each point should be contained within a separate paragraph.

It is always a good idea to express each separate point within a separate paragraph because this allows the reader to skip through your letter [or post] more easily, reading only what interests them.

Obviously, different readers will have different interests and different levels of interest. This last statement is more true of a "post" where you are writing for a very diverse audience which is unpredictable because it is always expanding [hopefully] than of a business letter composed for a particular business with narrow and predictable business interests.

To make it even easier for your reader to skip through your long letter, you might consider using subject headings to introduce each individual point.

When writing a post, one must always remember that one isn't writing only, or even primarily, for the people who are already posting. One is writing in the hope of attracting new interest from passers-by.

Posters who write only for themselves, so to speak, limit their audience, virtually forever, to their existing audience. This may suit the existing audience but it denies the forum the opportunity for growth, at least vigorous and continuous growth.

The "majority" in a dynamic online forum is not the "majority" which exists, but the "majority" which could exist if the current majority opened itself up to an unlimited audience.

Of course, this expansiveness could come at some cost to the current majority as some of the individuals composing that majority, or even all of them, might end up outside the new dynamic majority, subject to the ability of each individual within the original majority to compete on his or her own merits with an unlimited infusion of new talent.

There's the rub.

The growth potential of an online forum is limited not by the vision of the forum, but, instead, by the courage and self-confidence of those who compose the current majority.

If the current majority is determined to remain "the only" majority, a dynamic majority, and, therefore, a dynamic talk forum, is not possible, even theoretically.