Incidentally I've been considering just such a metric—one that would intuitively relate a sense of the user's overall progress to the next ranking, how much work they did to get to their current ranking, and yet be simple to calculate. I call this invention "post count." The patent application has already been submitted.
(While we're waiting for the patent to be awarded, I've updated Ness's post to include the actual thresholds. There's also another scheme that people with no user profiles are forced to use, based on powers of ten: lurker (0), vagrant (10), outlaw (100), vigilante (1000). Hopefully no one ever gets to "vigilante.")