Well, I honestly will love to know this as well, and I hope that admin or anyone else who knows will come around and explain this better, I personally used to think that ranking up was measured by a number of karma and activity a user has, like its done on bitcointalk where activity and merits earned are used to rank users, but it seems like I was wrong afterall since some one told me (I think last week), that karma is not used to measure a users rank, but only activity is used.
I will personally like it if admin can write a comprehensive details on this and pin it to the top in the beginners board for easy reference to both old and newbies, Ive actually checked and can't find any thread with this information on the list of pinned posts on that board.