Some of the sites that do "gradual engagement" piss me off when they let me believe I'm about to do something and then surprise me with a sign up form. It strikes me as tricky and I very frequently walk away.
I certainly believe it's smart to let users do as much as possible without requiring them to sign up. I think the idea of signing up is pretty ingrained in people though. If you make it dead simple (like this site or reddit) it's not much of a barrier and it keeps things straightforward.