2. My major concern about any mix-in kind of design (including inheritance and roles) is that people can wind up having methods available and have no idea where the method came from. I'm hoping that there will be a limited number of very useful traits and roles that are built in (or at least widely used) and people don't go about building too many of their own. That will let me amortize the cost of learning them over a lot of usage. This is not, however, fundamentally a language design issue but an education one.
I expect this one to be solved culturally, as you say. Plus it will help to have tools to tell people where their methods came from--when they want to know that, and not sooner.
