Once there is a thread already, users can write their replies in message editor and simultaneously watch how their reply would look like when posted (i.e. it works in WYSIWYG mode).
On the other side, when user wants to create new thread (start a discussion) then he/she has only standard editor available. There is no WYSIWYG preview, just plain old text (markdown??).
So for the idea (or maybe bug) - any chance for preview of the actual OP when starting a discussion?
