This is the first StackExchange site I've been involved in as a private beta. I've heard murky rumours that not all sites make it past this phase; how can I understand what criteria this site will likely have to meet (either quantifiable or not) to graduate past the private beta, assuming that step is indeed not guaranteed? What should we as a community be focusing on to make sure we meet those criteria?
Three big factors:
Are folks using the site? If no one is participating now, then opening it up to a wider audience is pointless; the folks who committed to participate in the private beta are assumed to be the most interested in seeing the site thrive, so if they're not using it then there's no reason to continue.
Is the quality high? Are questions focused and clear? Are answers well-researched and comprehensive? Are real problems being solved here?
Is the topic filling a unique niche? This is particularly applicable for Vi/Vim: there's a tremendous amount of overhead involved in operating a separate site, both for Stack Exchange the company and for the community involved in day-to-day maintenance. So if the majority of questions asked would fit right in on an existing site, then we're better off folding it into that site and letting the group that created it reap the benefits of being part of a much larger, richer community.