When you decide to not only use an open source software project but to really rely on it, accessing community viability is important. You don't want to build your business around a project only to discover that nobody is really working on it, bugs never get fixed and there are no new releases.

At OpenLogic we've created a number of ways to measure a community's viability including things like the size of the developer community, number of code commits, bug fixes, forum posts, etc. However, one thing that is hard to measure is the importance of the first lead developer. I think once a project has been handed down to someone else, this is less important, but the first lead developer plays a crucial role in a project. As Paula Rooney points out, the Reiser file system may fail if Hans Reiser goes to jail. There's nothing wrong with the file system and it has an active community but it's still being led by its first lead developer and if he drops out (especially to negative publicity), the project may not keep its momentum.

Once a project has moved on to a second lead developer, with the blessing of the first one, I think the project is much more stable and less dependent on the lead developer.