PG has been given [literally] dozens of great suggestions like yours, yet has not implemented them. I think that he is trying to cultivate a respectful community through conversation (i.e. comments), instead of applying technological solutions.
When your entire site is a technological solution, that doesn't work. It's not natural to divide conversations and rank them based on merits of "good" and "bad". This is why flat conversations don't succumb to groupthink as quickly or as badly.
I'm definitely going to change the site. The reason I haven't yet is that (a) I haven't decided exactly what to do, and (b) a new batch of startups just arrived at YC a couple weeks ago.