Filtering comments by account age sounds a bit too xenophobic to me. However, your idea of an apprentice system is interesting. This is a random idea based off that, but what if new users were given posting privileges only after demonstrating that they can recognize quality comments by upvoting them? Since HN no longer shows vote count for comments, if a user is able to consistently recognize quality comments, then there is a higher probability that he/she will also contribute quality comments. This would effectively require users to read and learn HN posting etiquette/standards before they can start posting themselves.
Intriguing. You'd need some way of providing feedback to them if they 'picked right'. Combining our ideas, this would mean 'picked like long-time users did'.
Re: xenophobia, it's not precisely that, as I'm not suggesting a date-joined-cutoff, just that it takes a long time for HN to be convinced you should contribute, rather than just learn.
