The importance of rattling your own cage
I saw "The Way Way Back" and was also pleasantly surprised. It's jarring to see Steve Carrell as an outright villain, instead of the uptight well-meaning doofus he usually plays. That brilliant opening conversation (which comes to define the central conflict of the movie, as the young protagonist makes clear in a heartrending later scene) was especially effective because Carrell's heartless "I think you're a 3" evaluation was as shocking as having him pull out a gun and shoot someone.
It's a clever script that never loses sight of the central premise you identified: the importance of rattling our own cages from time to time, and avoiding the kind of rut every major character is stuck in. It's an interesting contrast between the way Sam Rockwell's funny, groovy water-park owner represents both a guru of relaxation and emotional liberation to the protagonist, but is also an arrested adolescent trapped in his own inescapable life-cycle. There's humor in the way Rockwell and his employees accept their lot in life, but it's a bit sad to think of how things will be going at the Water Wizz the summer after the movie ends... and the one after that... and the one after that.
To the wise adult, children can be an early warning system against self-obsession and inertia. If your child is feeling moody, isolated, and abandoned, you're doing life wrong.