Both conjectures bear out in my experience. A third one:
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).
Your own profile provides an excellent example of this phenomenon -- you have a "must-read list". Valuable contributions from the past mean certain people get noticed more in the present.
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.