I used to be a huge proponent to single click sign in, in theory it's great. The problem I found with my own startup was that by allowing Twitter, Facebook or Google+ sign in, it was a point of confusion for the user. The amount of duplicate, even triple accounts was far higher that what I would have expected.

After reviewing the pros and cons, I switched to a simple email/password combination which also solved another problem of having to ask the user for their email address.

There really is a need for a true single sign in provider, in which you link your identity accounts to one 'super' account and then sign in with that, allowing whatever information is available from each as you wish, or simply a blank profile with only your identifier to link back to you.