...have a clear function; and the most useful and/or unique function (doubtless utility and uniqueness are linked somewhat!) should be presented first/earlier than other functions (to paraphrase Richards: 'one form for one function for as long as is pedagogically possible'). There is maybe no need to explicitly study separate forms that later overlap function-wise if some thought has been given to their ordering, primary function-wise, beforehand.
...(following on from the above point) also have a context that the students can relate to and will therefore appreciate the usefulness of, remember, and indeed be themselves thus using at some future point (perhaps quite soon!).
PLEASE FEEL FREE TO CONTINUE THE LIST! (I've started with some of the more obvious points, lazy of me I know, sorry!).
