I really think both you and Bruce are off-target where it matters
here. The vast majority of users, the people who are highly unlikely
to be using something other than Outlook or AOL's mail client and are
just mailing back and forth with people at work or family or friends,
are also rarely in situations where anything that we're discussing
even matters to them.
If you look at the primary audience for this sort of feature, namely
fairly sophisticated mail users who are on a lot of mailing lists,
mutt is*extremely* common. If you're trying to come up with something
that would be useful for the average Outlook user, I think you're
wasting your time; they don't use e-mail in a fashion as to make any
of this relevant to them.
It might be the case that only a small percentage of users participate
in email lists. It might also be the case that mutt is favored by a
fairly narrow class of users who happen to participate on lots of email
lists. But I don't believe it's reasonable to conclude that a
significant percentage of people who participate in email lists use
mutt.
Nor do I believe it is reasonable to assume that Outlook users are
unlikely to use mailing lists. I participate on several non-technical
email lists that have Outlook users on them. (as well as AOL,
hotmail, yahoo mail, and a surprising number of gmail users). And in
my experience these people do have strong opinions about where
replies should go (no, they don't all agree).
I don't see "fairly sophisticated users who are on a lot of
mailing lists" as the primary audience for this sort of feature.
(where "this sort of feature" is a general improvement in how
replies are handled and not something that is specific to lists)
I see the audience as "users who use email for more than
just occasional personal correspondence".
Keith