I think what boosted participation so much this year was that people had an option to be competitive in their RP, to feel like they had a stake in what was going on. An April Fool's joke can be funny, but with something like the Ungulate Revolution, people felt that (within the "story") they had something to lose. It was all in good fun, of course. I don't want to imply that competition is the primary ingredient for a fun April Fool's setup.