I see your point. I'm still not sure how it would be implemented in a way that the signal/noise was high, that the associated articles were relevant.

I run http://hubski.com that was built on top of news.arc. One of the things that I did to help tie posts together was to add tags. It's not perfect, but it does help pull up similar articles. You can follow tags too. EX: http://hubski.com/tag?id=architecture

It's a tough call. HN does work for most people, I think. The maybe the potential blowback is not be worth experimentation at this point? PG seems willing to make changes, but he's conservative about it. Maybe from lessons learned?