Jody Garnett, in "Open-Source Factors of Success," documents the three rules to a successful project: Do something worthwhile, accept contributions, make contributions easy. In true blog-style, he lists a fourth important section, "Documentation and Learning Curve."
Most Open Source projects simply have to follow a basic formula:

  • Do something worthwhile
  • Accept contributions
  • Make contributions easy
Now this is only a rule of thumb, you can make something useless and easy to contribute to and still have a good time. There is utility in fun.

I am tempted to list an Open-Development process as essential as well (it seems to be for making a community, but not always required for a project). I am going to put it down as a tool for building trust which goes towards making the decision to contribute easier for people.