Tl;dr; goal, feedback and public benefits of the initiative are a sufficient minimum to activate the users.
Goal-based initiatives that provide users clear feedback on the work they and its quality combined with a socially useful end result are sufficient for most initiatives to get users to participate.
There are a number of other motivators that are not in themselves the purpose of participation on the site as such, but they certainly make an additional positive contribution to stimulating user activities. For example: reputation points, public gratitude, extended privileges in the community, swag, etc. When thinking through your initiatives, focus on the essentials, setting all the auxiliary motivations aside.
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.