It is a perpetual puzzle to me why learning programming isn't considered to involve learning about functions. In my experience the hardest thing about programming is teaching people the facility of "uncoiling" in their heads the coiled representation of a loop or repeated calls of a function. That in turn requires as a prerequisite the ability to consider a function being called with multiple different inputs, and hold multiple possible situations as you build the function. Until you attack this, the hard problem of teaching programming, anything you come up with will only help learners with the first few hours of their learning. Which is invaluable, don't get me wrong. But what then? There's got to be something better than, "ok you're done with blocks and turtles, now go learn JavaScript."
(I have a start of a solution to attack this problem: http://akkartik.name/post/mu)