Any successful open-source project needs these things:

1. Optimal market timing (solving a real need in the market);

2. A strong, inclusive team of developers and non-developers;

3. An architecture of participation (more on that below);

4. Modular code to make it easier for new contributors to find a discrete chunk of the program to work on, rather than forcing them to scale an Everest of monolithic code;

5. Code that is broadly applicable (or a way to reach the narrower population more niche-y code appeals to);

6. Great initial source code (if you put garbage into GitHub, you’ll get garbage out);

7. A permissive license—I personally prefer Apache-style licensing as it introduces the lowest barriers to developer adoption, but many successful projects (like Linux and MySQL) have used GPL licensing to great effect.

Of the items above, it’s sometimes hardest for projects to actively invite participation. That’s usually because this is less about code and more about people.