Brad Feld has the interesting insight that a content publisher should worry most about the 19% who are left after the 1% of people who will actively participate in any new online venture (give up on the remaining 80% who are contributors only and won't actively participate). He also points to Tom Evslin's site where you can even download an Excel file to model this.
My quick take: you engage the 19% by offering them a way to participate that is brain-dead simple. For example, Amazon's "Did you like this review" feature--just click yes or no and you're done. No registration, no typing, just click on a link and you're done.
I also think the world needs something to expose your passive activities (e.g. surfing from link to link) in a way that helps you build content and maintain privacy without specifically needing to enter something.