When people call a work of art “trash” (any kind of art), they typically mean that a piece doesn't observe the basic principles of that type of art, that it is shallow, factually incorrect, or stupid. In short, the author's intentions were poorly executed, the piece is of low artistic quality, and thus it is "trash." Since we are talking about art and not biology or astronomy, arguing that a piece is good when the majority of people deem it bad is no crime. Argue away! But you might still be objectively wrong. If you are trying to argue, for example, that a one-star horror flick was just as good as the exquisite epic that just won an Oscar, well, you’d simply be wrong. You’re entitled to your opinion, of course, but that doesn’t make you less wrong. Objectively speaking, the epic has a more complex, profound plot, state-of-the-art cinematography, the most experienced director in the business, nuanced acting, and so on.
YA literature is no different. Take the lowest quality novels reviewed on this site and compare it objectively with the high-star novels (such as Between Shades of Gray), and you just won't win if you try to argue that neither/both are trash. The low-star novels tend to be shallow, silly, simplistic, with artless language, one-dimensional and/or uninteresting characters, and an insubstantial plot. The high-star novels are rich in beautiful language, profound themes, powerful and complex plots, and memorable and believable characters. Thus, we can say objectively that the one-stars are artistic "trash" and the five-stars are "gems."
And yet...and yet this will still not keep some from calling the low-starred novels "good" and the high-starred novels "trash." Why is this? And are they right? Of course, we have already noted that it is a person's moral judgement that often results in a "trash" or "treasure" assessment. Some people do judge a work of art from a moral perspective; the artistic qualities are secondary in importance. If it doesn't make the moral cut, it is trash--period. For others the artistic and subtextual qualities are of primary importance. In their opinion, moral/disturbing issues that arise in the work can be discussed, pondered, and judged, but they don't (and shouldn't) define the quality of the work.
There is another way people judge art, though, and it has to do with the emotional power of a work. I am thinking of a well-known television show from the '70s and '80s: Little House on the Prairie. As a child of the '70s, I grew up with this show. Every weeknight my mother and I would eat our dinners in front of the TV in our darkened living room, following the adventures of America's best-loved pioneer family, the Ingalls. We laughed, cried, worried, and cheered with them. And because the rest of America did, too, the show hung on for nearly 10 years.
Not only was the show fun to watch, meaningful, fairly complex, as American as apple pie, and morally wholesome, but it was emotionally powerful. I usually ended up trying not to cry by the end of the episodes. These characters were real, and I don't mean as in history (which they were, of course). I mean that Michael Landon, Caroline Grassle, Melissa Gilbert, and all of the principal actors became the characters they played to TV viewers. Who can imagine Charles Ingalls with a beard, as a quick internet search will show he had in real life? Ppfft! To me he and the rest of '70s America he was a buff, handsome charmer with curly, black hair. Yes, these people became friends we came to love (or love to hate, in the case of Harriet Olsen). Like the velveteen rabbit, these Hollywood constructions became Real.
But five-star cinematic art it was not. The plots were sometimes thin and ridiculously unrealistic. The characters were often melodramatic (I mean, couldn't the kids ever think of some way to express their hurt outside of running away?) Sometimes the show's overall themes reflected modern concerns a little too obviously (teen drug abuse, anyone?) Though the buildings and costumes look good to me, the general setting about as un-Minnesotan as you can get (and yes, I have seen the real Walnut Grove). And yet...and yet I couldn't care less. Maybe artistically speaking some would argue that Little House borders on trash (and maybe even crosses over), but emotionally? It is one of the best television shows ever produced, and I have no doubt that an enormous host of fellow viewers would agree with me. Sometimes neither artistic or moral aspects have little to do with how we judge art; sometimes, it's how deeply it touches us.
If I had more time, I could argue for cultural standards could be another valid vantage point from which to judge literature. After all, there does still seem to be some general consensus out there on what qualifies as child- or family-friendly. Morals play a part in this, of course, as well as patriotic and age-appropriate standards. But this is enough for now. How do you judge whether a book is "trash" or a "gem"? Or do you see it differently altogether?