I like your definition, but I think you're missing something
in it: objects. Actions are performed on objects, and the
way a user conceptualizes the objects they are acting on to
achieve goals is an important part of their expectation of
what the results can or will be. So I would say:
Goal + Action(s) + Object(s) = Expectation
You could collapse Actions and Objects into the term Message
that you use, however, I dislike this because it seems to reduce
interaction to communication (or behavior to meaning, as I've seen
some IxD theorists do). That isn't (IMHO) appropriate,
because taking actions on objects can change both the objects
and the actions. The real world is directly affected, beyond any
communication that takes place.
My two cents.
Manager, User Interface Design
Framingham, MA 01701
If any of you are particularly interested in the subject, I'd love to
hear some feedback on whether you think I'm dead-on, dead-wrong, crazy,
or have muddled things when I was trying to make them clear.