Tl;dr; Fear and hate are the strongest human activators.
Fears and hate are very strong emotions. They should be used in exceptional cases. If you feel that it is the case, try to find topics that are based on the users’ desire either to avoid a future that scares them or change the present reality that they do not like much. Initiatives proposing work on such topics are effective in activating users and are compelling in their own right.
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.