It's so easy to think of exceptions to these definitions that the definitions start to hurt my brain.

I don't think who sets the objectives is the defining element in the three categories. If I make up an objective for myself and actively pursue it through a book but don't have the self-discipline to work through the book and then sign up for and complete a university course on the subject, what is that? Surely it is all three. If I am forced to take a course but am surprised to discover a deep and active interest in its goals, is it still just formal? Does only accidental learning (ouch, that frying pan was hot) count as informal?

In that sense, these categories are no more useful than saying "a course", "a book", and "a hot frying pan." (Forty years later, my mother loves to tell the story of how I learned not to touch frying pans... hmmmm... maybe that unfortunate day was a non-formal situation engineered to achieve her health and safey objectives for the family...?)

In my opinion, the difference between the three categories is simply the adminstrative effort involved in each, and the fact that somebody in addition to the learner cares about the learning. The learner may share these goals or may not... but it is the presence of the other (person or organization) that makes it formal or not formal.

As a sometime instructional designer, I find it most useful to think about how much work and mental effort I suspect the learner will devote to the subject. Here, the categories of formal, non-formal, and informal do indeed help.