> and also that there is a left- and right-identity 'return: a -> f a'

RTF, uh, Comment.

The point being that I think it's best to understand return in the context of the more important operation join. You might wonder, "well why don't functors have a generic constructor", and (among other answers to that) the reason is that it doesn't serve any purpose in the functor laws. But return does serve a purpose for the monad laws, namely, being an identity of sorts.