To answer this question one needs to answer the following: What does health or success mean for the entire project?
My answer would be:
A healthy project is the project where users efficiently get what they came for.
Any community project should have one composite metric that incorporates efficiency of all main mechanics on the site. Company executives and department heads are the primary users of this metric. In many cases, this metric is accessible to every employee in the company so they can see how things are. The metric should be extremely simple to present and interpret.
Top level metric is unique for each project, though the idea behind is the same. Any community is a tool that solves a specific problem. The top-evel health metric should show the effectiveness of the project as a solution for the problem.
This is a fragment of a draft of the book “Lessons Learned While Working On Stack Overflow”. Read the full book on kindle or the paperback version.