Wherever possible, we are working with the Stack Exchange 1.0 sites that have created successful communities. Generally, it is better to bring them into the Stack Exchange Network rather than shutting them down or creating competing sites. The more people we get together in one place, the better the site will be.
But there are very, very few sites that were successful under the SE 1.0 subscription model; even fewer that would qualify with sufficient traffic and activity to get out of the SE 2.0 beta.
The best way to show that a site would be viable part of the SE 2.0 network is to create a corresponding proposal in Area 51. That establishes the minimum requirements that you have an enthusiastic, committed group of expert users who will check in regularly, asking and answering questions. That's the purpose of Area 51.
When sufficient interest has been shown in the site's topic, we have to look at the SE 1.0 site content. It has to be compatible with the way the proposal was defined and evolved. It also has to have quality questions worth importing into a brand-spanking-new site. Many of the SE 1.0 sites were not maintained and have succumbed to spam and off-topic posts.