Optional signup with the big three (Facebook, Google+, and create-your-own-email). As for the limited feature set before choosing one of those three, I used to

Optional signup with the big three (Facebook, Google+, and create-your-own-email). As for the limited feature set before choosing one of those three, I used to think that was necessary, but I've noticed more and more people don't care as long as they can choose one of the big three. Especially on mobile where typing passwords, etc, is difficult. It is much easier just to click the Facebook or Google buttons.