the reason amazon has a better ux is because it asks for authentication only when necessary. sure, you can add anything to your cart without a password, but you can't buy anything. the account security is still password-based.

sites like quora suck at this because you have to sign in/up to do anything on the site, even though there's no private information being accessed.

the problem isn't passwords - it's how we're using them.

I like this concept and I already though about it. Specially in mobile it’s easy to do so.