I believe you are essentially right about this. Leaving aside the fact that the quality of comments left by the "old guard" is not always in proportion to the u

I believe you are essentially right about this. Leaving aside the fact that the quality of comments left by the "old guard" is not always in proportion to the upvotes they get, I agree that as HN grows the problem of quality control is essentially a problem of karma inflation. With this inflation, the weight of an individual vote lessens dramatically and it becomes especially evident in the article submission mechanism where there are no downvotes at all. Encountering a user with less than a thousand karma points today is exceptionally rare. That's why 20 points feel so arbitrary. The more I think about this, the more I believe the karma system itself is vulnerable to issues scale. There are a number of ways to "fix" this, but I don't think any of them will actually be implemented here because they would change the nature of the community too much.