Think about this: you can’t build ANY service that needs identity if the service usefulness is smaller than the cost (mind and time) of the login process.

– Davide ‘Folletto’ Casali answering “What’s wrong with OpenID?” on Quora

What a perfect way to phrase it. Apple’s various app stores have been so successful because they have lowered the obstacles to app discovery and installation enough that a whole new class of focused applications suddenly had greater usefulness than the cost of finding and installing them. There is an opportunity equal to or greater than the app store out there for web applications.