The (volunteer) moderators on the site are

Bingo. A (well) moderated group will not have an off-topic posting
problem because those off topic posts will be moved or labeled, and the
subject lines are likely to remain useful. The off topic discussion can
continue but the whole thing is far more organized. Absent moderation,
individual posters have to do it themselves. Cats are easier to herd.

But if we want it, we can be more proactive with subject lines and
prepends. We'll accomplish the same result, and do it on Usenet,
without resorting to another group.

What I think happens is that posters in a
newsgroup like r.a.piloting see that other posters mostly share their
views on the main topic (aviation) and then assume that other posters
will also mostly share their views on other topics, including
controversial ones.

I think there's also the fact that the OT topics that come up here have
profoundly affected aviation, and are thus of special interest to
pilots. Also, as we get to know each other, we are interested in each
other's views on things. Personally, I am not at all surprised that
other people don't agree with me. After all, I =am= the most
enlightened one. g,d

I'm not sure if this is
something you will ever be able to fix in software.

No. Just wetware.

Putting things like "OT" or "POL" in the
subject line works for a little while but then gradually becomes
ineffective.

I think we just forget. We can all be more aggressive in changing
subject lines. How does this affect (or break) threading?

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
for Email, make the obvious change in the address.