Last fall, I was on a panel at a convention, and the topic of the convention was "What's too scary for YA"? I think that was it, but if not it was the gist of it. My answer was, "If you've read THE HUNGER GAMES, pretty much nothing." It's kind of a flippant answer, but the audience laughed and that's what I was going for.
But it's a valid question. There's nothing 'out of bounds' for YA literature; if you've read widely, you know this. I once had a publisher decide not to release a series of books I had worked on freelance, because the main source of magic was a weird kind of shotgun. The series was TERRIFIC as far as I was concerned, and I tried like anything to get them to put it out there. "Have you seen what's on the YA shelves? Have you physically gone to the bookstore and looked? There's drugs, suicide, rape, death, cutting, abuse. These books use guns to shoot magic -by YA standards they're practically tame!" But in the end they did not put them out there. A shame, because they were really wonderful.
Not that all YA is like that, of course. There are plenty of stories that have hard choices but no squick, some that involve emotional choices instead of physical ones. But YA writers certainly don't shy away from difficult issues or events in their books. Bad Things happen in Upper-Middle Grade stories too, and in adult stories. But if I were to write about a death, for example, you would write each differently. Murder happens in MG books, but you probably won't see it, it will happen 'off screen', and the reader will hear about it after the fact, or if a car goes over the cliff and explodes, we can assume the passenger is in fact dead, but the author might not write about the flying brain matter or the smell of burning hair. In YA, the murder might happen 'on screen', and it will be a little more graphic but an explosion might include the more detail. In an adult novel, well, if you've read Stephen King (which I read in High School, but that's another story), then you know what I mean.
Just like writers shouldn't turn their characters away from the hard choices, they shouldn't turn away from the tough scenes. If someone's going to die, they need to die. If they're going to be in a fight, or assaulted, or stabbed by a letter opener, they should be. It can happen on the page and not be 'too adult'.
In my novels, I've had people die in nasty ways-- being eaten by a Minotaur (happened in the dark while people ran away, no one actually saw it), sucked into the floor of an Ancient Egyptian temple, and nearly fall over a cliff. I had a main character chop off a monster's head. And these are TWEEN books. With a few well-turned phrases and a character that shut her eyes, the scene was perfect; in fact, one Amazon reviewer said the book was 'surprisingly clean and gore-free', even though the mythology I was using was decidedly...not. The 'squick' was part of the story I needed to tell, and skipping it would have meant I was shorting my readers and my characters, denying them the payoff and the thrill of the adventure. It would have meant that I was the coward. If it's too easy, it's not worth the effort of reading. If the same book was YA, I probably would have written that scene differently. Probably.
Bad things happen to good characters. Sometimes really bad things. Just ask Katniss. But, a good writer will allow those bad things to happen without making them sound either less frightening or important than they are or too graphic for the intended reader.
Because every story needs a little surprise, doesn't it?