when moving a thread from a forum to another, the moderator must add a post in the thread to briefly state the reason. when deleting a thread, it is a courtesy to inform the author by PM. some ET moderators do good jobs. these ideas came from what they are doing already.
this is a little different because it's something that would be built into ET's code, however... some message board sites insert a message at the bottom of posts automatically when they have been edited by a moderator. i have had many messages greatly edited and no one but me and the moderator knew it was changed. that's kind of annoying.