I've done loads of critiquing, too, and here's what I've noticed:
1. No voice. There's just nothing grabby about what the character is doing/saying/thinking. Kids are amazingly quirky so there needs to be some of that in the writing.
2. Starting in the middle of the action with not enough back story to make me care about the character. It's a tough line to walk, though. See number 3.
3. Too much back story and no front story. It's all taking place in the character's head and I have no idea if said character is in their room, in their treehouse or on the moon.
4. No established internal arc. After reading the first chapter, I want to know what the story will be about, but I have no idea. I might know it's going to be a story about a character on a quest or somesuch. But I have no idea what is driving the character to the quest, what their internal motivation might be. The wound they need to heal.
5. Self-conscious writing. When the writing is so concerned with the above and/or other people's pet peeves, it strangles the life out of it.
I think good writing comes from letting yourself play. Even if it's a tough subject. And so much of what I read is just, so, ADULT. Rational, reasonable and well thought out. There's nothing spontaneous or joyful. I LOVE that Kurtis's boy discovers the runaway because he's taking care of someone's PIG. Sorry for all the capitals.
PLAY! Let's PLAY. If that means the kid is uber quirky and grandma rides a skateboard - GO FOR IT. It would be eight million times better than reading about the kid who eats eggs and toast for breakfast and then goes to school. You could and probably should take the skateboard out of following drafts, but going overboard in the first draft, I think, makes the writing more flexible and flexibility is where the gold is. Writing about grandmas on skateboards and kids named Alcatraz can warm up the muscles and make the writing fun. Then you can revise out the overboard stuff, or make sure there are good reasons for the weirdness.
Sorry if that was kind of ranty.