[mjn]

I would guess it correlates highly with number of posts, so I just think of it as a proxy for "highly active long-time users". Another measure could just be postcount. Karma is like a slightly "corrected" postcount that is supposed to boost people who post highly regarded posts, and downrank those who post a lot but mostly junk. But I would guess just raw number of posts is still the main explanatory factor in the ranking (I don't have the data to test this guess, unfortunately).