This would give comments context. The site would in effect be saying "hey, you've read four or five comments by this person and thought they were sharp." or, "don't waste your time with this comment, you haven't liked their others." I don't know how many times I've read smart comments without actually connecting that they were all being written by the same person. It is only extremely good and prolific people who I actually recognize by hnname. This would help me find more.
This is really a reputation/karma system, but scoped per user instead of site wide. You can go further and trickle votes down the follow chain, so that the people who I follow follow also are part of my personal reputation network. This would help cut down the amount of interaction I have to do to make the following system useful. This is essentially page rank.
With this in place, HN can become a more personalized aggregator wherein the links and comments that are liked by the people you like are more often presented to you. It is quite possible this could create the equivalent of subreddits organically as the site's membership creates following chains interested in different things.
Now, this is a very technical solution to the problem, which means it probably isn't merited. I think that metafilter is probably one of the right guides to watch and that for them careful moderation has been key.
Also, there are a number of real problems with this solution, the first being that it significantly increases the risk of the echo-chamber as people start to be split in to like minded groups. I've thought about some ways to deal with these issues, but I don't feel like this post is the place for them.