I was a part of the Big Data beta about a year ago and I was saddened when it didn't meet the qualifications to get out of Beta.
I have been a part of 7+ betas so far of which a few were closed. It is always disappointing to see that happen. However, the decision to close a site that is a ghost town is much better than keeping it open and causing frustration to any people who may come across it in the future.
One other thing that I came to understand is that every beta is different. The metrics are variable, and are not applied as-is to every site. Various considerations such as engagement, moderation, commenting, and quality come into the picture; none of which can be easily given a number.
We need to average at least 15 questions a day.
During the beta, it is more important to create 4-5 good researched specific questions per day; than 15 mediocre ones. For the first few months, quality is paramount as it sets the tone for the site for the months to come.
We need to average 90% answered questions.
This is another parameter that varies depending on the community. In some cases, it is simply not possible to maintain very high ratios as experts of certain niches within a field may not be present. This doesn't seem to be a problem here; the current percentage of answered questions is 100%.
Keep coming back and encourage all of your friends to keep coming back. We should have 1500 visits per day. With Big Data we only managed 60/day.
This is something that requires attention and directed efforts to increase. The visit/day is very low. It deserves a separate post, though.
There are a couple of checkpoints where Stack Exchange will pop in and decide whether or not to keep the beta going. I had originally thought we had 30 days to get things going, but we were shut down after 18 days. I've seen another beta shut down after just a few days, although I can't remember the name.
The policy for the past year has been to have 1-2 week long private betas, after which the site is closed or made public. This site got an extended private beta and is one of the first this year to have that. The 30 day beta period has not been the policy for over a year now.