Reddit reviews Girls Like Us

I think voice and tone are the main markers of YA, and those are incredibly hard to nail down.

The Curious Incident of the Dog in the Night-time, The Ranger's Apprentice, My Sister's Keeper, Miserere, The Midwife's Apprentice, The Catcher in the Rye, the His Dark Materials trilogy, Ella Enchanted, Catherine, Called Birdy, Fangirl, the Mistborn trilogy, Girls Like Us, various Tamora Pierce books, and Incarceron are all books that could be considered YA in some markets, but not in others (some are marketed up as adult literature, others down as children's books).

If you went solely by "characters being teenagers for most of the book" to define YA, (and even threw in caveats like "coming of age" and "no explicit sex") you'd get titles like Wild Ginger, The Poisonwood Bible, Balzac and the Little Chinese Seamstress, Into the Forest, or The Year of the Flood on the YA shelves, possibly disappointing a lot of people who aren't interested in such a dreary world view and often a pervading sense of melancholy (which is perhaps coming from the slower pace, even if things are happening all the time?).

Endings seem to play a role too: those adult examples were all unhappy ends that could make the characters' entire journey seem pointless. YA doesn't necessarily shy away from the unhappy ending (The Fault in Our Stars, The Girl of Fire and Thorns, and Feed come to mind), but there's always a spark of hope and the books were more upbeat up until that point.

YA doesn't necessarily shy away from cynicism or ennui and/or despair either: there was plenty of that to go around in The Hunger Games, Looking for Alaska, Graceling, Delirium, and The Archived, but those tended to be character traits coming from character voice rather than the tone of the narration itself.

Bonus MG vs YA distinction: Does he liiiiike her and maybe kiss her or marry her or are they dating or secretly lusting?

tl,dr: Gut feeling. I know it when I read it, and I don't always agree with the official designation on the spine.