This suggestion has been closed automatically because it did not receive enough votes over an extended period of time. If you wish to see this, please search for an open suggestion and, if you don't find any, post a new one.
Perhaps on a forum by forum basis, but the idea is you can have a new thread with the first post, and all subsequent posts take on a "Comment" format as opposed to looking the same as the original post.
That is more of an idea of a MOD. shouldn't be tough to do, in VB parlance you just have to consider the post number with an If condition and insert your code in the template hook. I don't know about the XF structure though.