As for edits. Algorithm::Diff could be used, or every version could be saved and appear as links. Editing info is not always a problem and without a preview it is practically necessary. (Consider noticing broken html tags.) What tends to cause problems is when someone points out a mistake, the mistake is edited out without acknowledgement, and then the person who pointed it out gets upset that they have been made to look like they are making stuff up...
As for deleted threads, deletes are quite common. Usually the reason is a duplicated post. Deletes that cause controversy are quite rare. The last time one caused an issue that I know about was a few months ago.
