What has to happen for this site to progress beyond the beta stage?
This site is currently in the fourth and final phase of an Area51 proposal -- beta. You can read more about this at the Area51 faq.
This is the relevant bit:
If the site does not get used, it will be deleted.
As our proposal page says, the site will be reviewed on October the 5th, 2010 (90 days from the private beta start).
It's not that clear yet what the metric will be to determine if a site graduates from Beta.
We are the second site to reach it, so we are kind of guinea pig in this.
In my opinion, the most important thing is traffic, we need to make the site popular and have lots and lots of views -- I doubt they're going to compare it with stackoverflow, but they sure will compare it to other SE 2.0 sites.