Luke Wroblewski evangelising gradual engagement:
Gradual engagement is the process of moving a user through an application or service — actually engaging with it, and seeing its benefits. With gradual engagement, new users are not just presented with a registration form and then dropped off a cliff. Instead, registration is either postponed, or handled behind the scenes and the first time experience is focused on giving people an understanding of how they can use a service and why they should care to.
The article is around four years old, but apps like Airbnb, Meetup, Medium and others are still guilty of forcing registrations upfront.