Would it make sense to permit people to reply to Forum posts directly? As in a private reply to the person who made a post rather than a public reply within the Forum.
This would treat Forum posts the same as stories which might be overdoing things, the next question would be whether to permit someone to block all such messages.
Don't know how this goes reader to reader, but for those of us who are authors and have feedback turned on it's already possible as I already get some responses direct by people simply going to my author page and using the email link there to send me a personal message outside of the forum when they have something they want to say they feel should be outside the forum.
Yes, that is obvious.
A PM system between all accounts, as long as the account holder permits it. Occasionally people post email addresses here, this would eliminate the need for that. People frequently communicate "privately" here - for all to see - and this would remove such discussions to where they logically belong.
I have little idea of the overhead involved, programming or database.
For those of us who have been around for a while, we have a LOT of author's private emails, so we can easily take public comments private (though that does little to 'tap down' inflamed rhetoric). For those without such background, it's generally fairly easy to look up an author's email, if they choose to make it available. For authors, it's not uncommon, after a few 'message', to suggest moving the discussion to email to allow for more substantial responses.
As Ernest Bywater indicated, this is more about communicating with those of the non-author persuasion.
We used to have that capability (on the old Google-based Forum). You'd just click a separate button and you could email the poster directly (useful, since Google required you to log in using a valid email). The poster could easily ignore it if they were uninterested, but it allowed us to take some of the more protracted discussions offline while we argued over details rather than concepts.