The biggest pet peeve of mine, I gotta say:
Pointless character deaths
Now of course that isn't to say that character death itself is always bad, of course not! There are ways of doing it that can be very effective and work for the type of story you're making. But that said, it's a really difficult needle to thread.
On one hand, you have characters that you can just tell were basically made to be killed off. Often times they're just a completely under-developed, angelic fountain of positivity, purely there to be a support for the main character so that when they're inevitably killed by the villain, the main character can have angst about it and be all NOW THIS IS SERIOUS. And yet, the audience is left feeling absolutely nothing for them because they never really got a chance to know the character, nor were they really given any reasons to care about them, so it's mainly just a feeling of "...well that happened."
On the other hand... you may have a character that an audience really gets to know and love, they have great development, you're really rooting for them... And then they die for the most pointless reasons, completely out of nowhere, purely for the shock value, and are basically never spoken about again.
I tend to see the former happen more often than the latter, but when the latter happens... If they're a favorite who dies a cheap death with no chance of bringing them back, I can guarantee that's where a lot of people will stop reading.
Now sure, this does depend on the type of story to some degree- like yeah if you're doing a zombie apocalypse story then people will pretty much go in expecting everyone to die, that goes without saying. But just... Basically what I'm saying is please treat your character deaths with care.