I can overlook "Bella Swan" characters for a good plot and fast-past, well-told story. But when that story is slow, tedious, and pointless, it doesn't matter how cool the characters are.
Nothing challenges Mary Sue; she somehow has a clever solution for every problem she encounters. She is always upbeat. She radiates positivity. She has her life in order. She knows where she will be in ten years. She fears nothing. She is BORING.
I need to feel a connection with the main character. It's why most of the books I read have female protagonists; I connect most with them. I have no connections to John Doe the brain surgeon who must solve his daughter's murder. I need characters with a familiar emotional chemistry.
Nightshade by Andrea Creme is a perfect example of this. There were so many YA Fantasy tropes, it felt melodramatic. Unoriginal.
I can't stand the preteen drama of Popular Girl versus the Nerdy Smart Girl, of Popular Guy who is for some reason best friends with Nerdy Girl, but is dating Popular Girl. I can't stand the "Who am I going to go to the prom with?" obstacle, or the "I'm not as pretty as that girl over there with perfect hair" thing.
Now, some of this drama sprinkled over a bigger piece of drama can add, but normally, I hate it.
This is the reason I stopped watching Game of Thrones. Ned killed his daughter's wolf, and I was done. Then I found out basically all the other wolves die, and I had no desire to find how out, when, or why. I wanted to send a bag of dog poop to Martin's doorstep. Still do.
This is a big one. Stupid sentence structure, repetitiveness, weird metaphors, stage-direction narration, no emotional connection, fan fiction type descriptions – I'm talking just plain bad writing. Unintelligent writing. Writing that makes Fifty Shades look good.