Imagine you have a visitor that will do anything you ask to get what you promise. What is the most lucrative way to setup a website with this assumption?
I guess it would depend on the value of what you promised.. You'd need to get him to do something which was > the value of whatever you promised him.. If what you are promising him is something such as recognition on a forum, i.e. awards next to his name, other vanity addons then you could easily create a business putting users to work doing a multitude of different jobs. Simlar to minuteworker but without having to pay them. You could sell your users combined services to other people, for example getting 100 of your members to go and sign up on a website in return for a gold star next to their profile.