I see two major barriers to mass adoption of any crypto system that requires a UI.
1. Abstraction. For the non-expert, the only metaphor that works for PKI is that of physical security. The concept of a "key" as a series of characters or a file that must be protected must be replaced by an abstraction that allows users to protect it in the same way they understand how to protect a key or a wallet.
So long as the "key" continues to appear to be an enormous password, people will continue to treat it that way, along with all the crazy worst-practices in the world. Not to mention the huge host of problems that accompanies key security for the layman.
I don't know what the answer is, but I do think that a physical token of some sort is the right start.
2. At the end of the day, the vast majority of people are not interested in the trade off between better security and convenience.
There is a learning curve, however shallow it can be made, and those that genuinely want or need better security are motivated enough to do the learning. For everyone else, it's like optional homework.
I would add to this that most everyone I know who cares about security is quite comfortable with a large array of APIs and software to do all sorts of cryptographic gymnastics. The bar is quite low already -- it has been for some time, IMO.