Know what I love about Rockwell? The hands. Artists always talk about how hard it is to draw hands. Rockwell, that dude could draw hands, and he knew it. He drew hands in this picture, through the doorway, when there was absolutely no need to, because he could. And if you look at a bunch of his pictures, he doesn't just draw hands, he draws hands doing complicated things, making complex gestures, gripping fiddly little objects, he draws old people with wrinkled skin and funky joints on their hands... he was goddamn good at drawing hands and he was not shy about showing off his hand-drawing talent.
People calling him an illustrator and not an artist are just jealous of his hands.
I think it's pretty well accepted that, even for a classically trained artist, the three most difficult things to paint accurately are human hands, a horse in motion, and the concept of epistemology.