Jared Spool gives an account of how changing a button made a big difference. Many site owners still measure their site’s success by registered members and that drives them to enforce registration. In fact a lot of user interaction is nowadays generalized assuming this. It is quite possible that the amount of information you end up asking users is almost the same. However, it is better to get a willing user to register instead of forcing him/her. In this case, I think that this was not just a UI change, it was a change in the policy. Of course a lot of wisdom here is out of post-event analysis, but this illustrates the importance of designing user interaction specific to the task at hand instead of generalizing it.