Each HN user is, literally, their comments + their submissions. This may be an anonymous forum, but people feel a strong connection with their HN identity. When they make comments that cause them to look silly, they feel silly. Why is that?
If you let people change their identity, people may subconsciously stop trying so hard to live up to their current one.
Maybe allow each user to change their username only once.
If deletion is allowed, it would be easy to add "undelete" from a previously scraped copy, or even a page for all deleted comments with previous user details. Both of these already exist for Reddit. In a way, deleting a post would highlight and bring it to the attention of people looking for controversy or accidentally leaked information.
I don't think so - it's clearly unreasonable to expect every instance of a username to be policed like that, particularly considering cut-and-pasting quotes. I'm fine with the idea that if it's in a thread it's no longer 'yours.'
But I also think it should be possible to remove ones' own account and have the username on those posts no longer point to an active account.
I also think there should be a "post anonymously" option as well, which there practically already is in the form of throwaway accounts. Might as well just make it official.