Often one big problem is that users feel attacked and answer in a topic whereas a private massage would have been enough.
If there was a flag option to move a message into a private conversation this would preserve that only posts staff public that are of a lot of public interest.
There even could be a heuristic, that tries to detect such posts. And a button for the author at the latest post to convert into pm directly.
Especially for new users such a button would be a big benefit for the forum