A lot of community-led projects I hear people talk about, locally or nationally, seem to be based around complex tech platforms and structures. Regardless of how awesome they will be one day, they spend ages in some sort of 'coming soon' or beta phase.
Impossible.com is a prime example of that - the small information that's out there points to an awesome idea that people will want to get involved with. But because it takes a long time to build a social network, I have to park my excitement when I get to the site, and wait indefinitely.
This is the tech that the Lambeth Snow Warden scheme is relying on:
It's just a pdf map showing where people are already involved, and where you can help. Once you're signed up, you're added to a google group to stay updated.
The main requirement in so many new 'big society' (cough) projects is a body of people. Maybe all you need, to start with, is an idea and a pdf.