Lazy registration and engaging users
Every hurdle a user has to hop through to get to the meat of what they’re trying to do on your site is another opportunity for them to opt out.
Lazy registration, where you get the minimum possible (dead link to webjackalope) from your users and get more information form them as time goes on, is where it’s at (in fact, increasingly technologies like OpenID and OAuth might even make lazy registration redundant).