It seems, as a Q&A topic moves from Propoal to Commitment to Closed Beta and beyond that, at each step, the SO system tries to further close off topics into niche community interest. Future users of the hypothetical site find increasing road blocks to involvement. I'll explain:
Proposal Phase Anyone can follow a proposal. Following gives the user easy links back to the topic, so they can keep connected, and involved to whatever degree they'd like. Unfortunately, there's very little they can do in this phase.
Commitment Phase Followers from Proposal are given the option to commit. New people can wander by and commit. There is no longer an option to sign up as a "future community member" we are now only interested in "future community experts". People who wander by and would have followed the proposal may not reach the bar for commit. Future users are rejected.
Closed Beta Phase No one can join. You missed the bus, sorry. There is no longer an option to connect with this community at all. No "notify me when this goes live" button, no "I support this idea" button, nothing. You will never be part of this community, because you were late.
As near as I can tell, there is no index of topics past this point. So, unless you were involved previously, it's effectively gone.
Public Beta Phase The site is finally, once again, accepting members of the public; but has hidden itself where no one can find it.
Is there a Public Non-Beta Phase? How is it different?
Recommendations There needs to be a way to connect as an interested party at each step, not just a raised bar for membership. The Commitment Phase needs a "I'd love to use this site, but I'm not an expert" option. The Closed Beta Phase needs at LEAST a "send me the link once this site disappears!" sign-up list.
I don't see why "Follow" and "Commit" aren't options all the way through to the end of Closed Beta. Following shows interest and support, and connects users to the future community. Committing indicates a level of support which I don't see why should be rejected just because someone noticed a site 24 hours after the Closed Beta started. However, I would support people only following not be allowed access during the Closed Beta, and late Committers having to be allowed access by current participants in order to maintain the feel of a topic.
Finally there needs to be a directory of live sites. If there's a distinction between Public Beta and Live, then probably both directories. But these need to exist. Otherwise, the meta-community that is StackExchange won't flourish.