I haven't any of the books mentioned by Stein but some of the most enjoyable books I've read recently have been young adult fiction. Yes, some are fluffy but some have complex characters and themes. This also holds true for adult fiction. Plus, "fluffy" books (I guess this is what Stein really abhors) are damn good fun. So basically, he should get off his high horse and let people read whatever they want to read. The world is such a better place when we don't judge people's reading habits.
Before I end, I have to say some children's books are pretty entertaining as well. I took a children and youth services course this semester and while doing a story time project, I read some children's books. Many are actually quite good and one even made me cry because it was really touching. So maybe Stein should stop dismissing books because of their age level and just appreciate good writing!