Perhaps there is an example or demo you could make public, to give less adventurous visitors (like me) an idea of what it does? (edit: like the /gauss link I just noticed - now I like it)
I'm all for stealth but IMO this is too stealth.
Building a bad community, not setting precedent, training up people and solidifying a culture are pretty lethal to your stated goal.
Once you are open to all, your constant battle will always be:
1) Not let the Signal to Noise ratio degenerate 2) Be able to split the Signal down into sub categories for people to process, as opposed to a torrent which then leads to arbitrary discarding of significant data.
With regards to managing signal to noise: 100% agree. Figuring out how to slice and dice the content so that you see what matters to You is our number one "tactical" objective. It is actually a hard problem, but one that gets easier as you come to understand the content being created and once you can benefit from enough liquidity to slice and dice at will.
We reasoned that the signed-in experience is a lot richer than what we could show signed-out on the landing page, but perhaps we should not take it to such an extreme :)