Come back to an old thread and it's full of [deleted] and "This comment has been deleted by a script blablabla by the way fuck you"
There's almost nothing about most threads that will be of any long term interest to anyone - so you might as well allow them to auto-delete after a certain time.
Maybe a good compromise would be to remove the user information after a certain time period (~2 years). Hashing the username salted with the post title would be a decent way of systematically respecting user privacy while also keeping old threads readable. I wouldn't mind if HN did this.
One day I'm going to run for office and I'm going to have to get lawyers to scrub HN of all my comments because they have no way for users to manage their content :-)
You could get away with something as simple as incremental ids in that case - user1, user2, etc.
Edit: for better readability it could be further mapped into a table of human readable handles, similar to how Google does the "Anonymous Lemur" thing in gDocs.
I'm not a lawyer, and admittedly haven't taken the time to look up the appropriate legal terms and other minutiae involved, rather relying on the kindness of my fellow commenters to extend to me the benefit of the doubt with respect to what I'm getting at, and helpfully clarify anything that needs to be. Thanks :)