A more cynical observation is that users with high "karma" point totals earn this by parroting the majority opinion or otherwise treating comment threads as a game.
There are many factors that could contribute to a power law for karma distribution. One conjecture is that other users tend to "trust" high-karma users and therefore tend to upvote their comments just because they have high karma.
Another is that high-karma users receive gratification from using HN and therefore use it more. Which leads to getting more karma.
Either of the two conjectures I advanced could explain a small number of users having a disproportionate amount of Karma.
High-karma users have followers. I've left comments on very old stories in out-of-the-way places and found them voted up. So we accumulate karma faster in part because people are simply more likely to see our posts, because they go out of their way to do so.
high karma people have a halo effect (while that sounds a little like one of the others ones mentioned) I think it's distinct.
I had an experience the other week with the halo effect as follows.
I was somewhere and struck up a conversation with a man that I believed to be the actual head of a top 10 law firm in the US (a distant relative who I met for the first time). With a private jet, home in Aspen and a few thousand lawyers under him. While I was talking to him, I was noticeably aware of literally how important he seemed on one hand, and how "down to earth" he seemed on the other. (He actually didn't seem that sharp to go with who I believed him to be, in a George Bush kind of way.)
But the halo was still there (I was doubting my instincts all along).
After speaking to him for 45 minutes or so I asked for his card. It turned out that he wasn't the head of the firm, he was the son of the head of the firm (he was a senior partner none the less).
So, to an outside observer, you might suggest that these users game Hacker News but the real answer is that they have an audience. It's debatable whether that's their fault, on a case-by-case basis, though.
Then you could probably say it goes the other way: a large part of the community feels a certain way because Ed says so (in the top position), or Thomas says so. I've noticed the atmosphere of a thread change after a comment from a "well-known" person is left, rather rapidly on occasion. The momentum of a community like Hacker News is an interesting study, and although I didn't get an opportunity to watch the thread under the microscope, I bet a large part of it was shaped by Ed's comment.
Recently I've learned how Hacker News threads are living organisms, and I've noticed the impact of the commentary that I make. If you pay close attention, you'll be surprised at how the thread evolves and grows, particularly based upon what's in the top position.
It took some work, but I managed to corner him.
I didn't want to lose this argument because he was trying to make me look foolish when I was technically right. Since he has a lot of street cred, I feared that a lot of people would take his ramblings as correct.
And some did. I got at least three down votes on the first message.