The devil is in the details, I think. What you're describing seems lik something that adds insecurity (single point of failure, etc.). The overarching idea is a good one, and if you can reframe it so that it gives comfort to technical users (who will then become your evangelists) you may have a winner.

I think ideally it needs to solve more than one problem, e.g. Make it easier for websites to handle user accounts or avoid having to handle them while not being Facebook or twitter.