With any cross-cutting abstraction like this, it's kind of like developing a new potting soil: I feel like I need to know what flowers you're trying to grow to understand why you made the choices you made.
I looked at existing mixin libraries, but found some that either injected themselves into the prototype chain or patched Function.prototype to let mixees customize behavior. I didn't like that, so I thought of this approach.
The rest has been a continuous process of simplification, but I never got back to writing that old application :).