a pm system could be good in some ways, but people will just use it for flame wars. and there would be no way for the mods to keep an eye on it, if tom adds a pm system, it would probably only last a year or so before tom gets annoyed by complaints and takes it down.
on the adultswim forums, there used to be a rating system, but after people abused it by rating low on a post just because they were pissed off at someone, it was taken away.
and an edit button, i hope that is never installed.
once again, using the adultswim forums as an example, the edit button is ment for correction of errors, but people mostly use it to flame others and spam then change their post. and even worse on those forums, there is a delete button, where you could flame someone, leave your post for 5 minutes, then delete it. that is the most abused thing on those boards.
so even though these things can be used to help us, im pretty sure that there are many n00bs waiting for the pm system and edit button to abuse and spread their internet jackassery with.