A feature that allows the creator of a post and/or the moderator of a community to reply to everyone who has replied to any given post. Universal replies will also be sent as email notifications to those who select that feature. Note that there have been several people expressing interest in a subfeature of this, which would allow the sender of the Universal Reply to select from amongst those who have previously replied, limiting email notification to just those users.
I think this feature would make communities a much more powerful place for planning, organizing, and scheduling, and it would reduce the need to take conversations to email, external mail lists, etc. This feature could help reduce the incidence of those multiple reply threads that finally align ludicrously over to the righthand of the page.
An abuser could technically post something inflammatory to a community, have a bunch of people reply, and then make a ton of universal replies, effectively mailbombing the people who replied to their post.
This, of course, wouldn't stop LiveJournal Abuse from suspending the user or prevent the owner of the community from deleting their posts and/or blocking the user from the community, but perhaps some degree of rate limiting for universal replies would be a good idea.
The Universal reply would (or should) be displayed as a reply aligned to the left, essentially in the same alignment as replies to the original post, and underneath the last reply made to the thread, but with some kind of icon denoting its universal status ( Perhaps an icon that looks like a pencil w/ swirl logo or a circular arrow surrounded by a bunch of our generic "people" icons, or a picture of a little pencil swirl around a globe . ) To perform a universal reply, you would view the original post and hit reply. If you are logged in and either the moderator for the group or the owner of the post, a section on the reply page will say Universal Reply, followed by a checkbox to implement the feature and a question mark icon which will link to an FAQ describing how this feature works. Clicking this checkbox, typing in the reply field, and hitting the Post Comment button will implement the Universal Reply.
There has been additional interest in having an additional screen appear after Universal Reply is selected for a message, that would display a list of all people who would be notified by the universal reply and either make the notification "friends only" or to have some kind of checklist of the people who could be replied to who replied to the original post, allowing the owner/moderator to select specifically those users who should be reached by the universal reply.
Those who elect to receive email replies should receive an email with the universal reply in it. When it arrives in email it should be designated with the same icon (for those who receive HTML emails) as well as perhaps specifying it in the subject of the message. "Universal RE : " This would allow people to do easy message filtering for stuff that isn't directly replied to them.
When a universal reply is made, there needs to be some kind of note that universal replies have been made to the original post, ideally underneath the original post.
For instance, you might see the original post saying:
Seattle Coffee Meetings
Would a group of LiveJournalers want to meet regularly each week for coffee in Seattle? Any idea where we should meet, and if so, how often?
Poll on suggested locations and dates
Poll results are in! First meeting scheduled!
... or something along those lines. Basically, the title of the Universal Reply would be displayed, linking to the full reply itself.