I love this topic. Venting about things I read in stories that frustrate me can probably be described as one of my hobbies by this point (plus I agree with nearly everything listed so far) x)
If the ultimate goal here is to provide writers with a list of things to avoid, there's a very general one that comes to mind:
Anytime a character makes a decision that seems very unlikely just to further the plot. I wish more writers, when writing a character making a plot-changing choice, would ask themselves "what would I do under these circumstances?", think it over as though it were a real decision that would impact their lives, and take the answer they come up with to heart.
This is not to say that characters can't make foolish decisions. In real life, we all make foolish decisions at some point or another. But I think there's something to be said here for stories being "larger than life". In fiction, unlike reality, everything needs a justification- including stupidity.
As an aside to that- I think love stories or subplots where a character is absolutely convinced they're in love despite knowing next to nothing about their love interest- or having almost nothing in common- are a good example of this going wrong. How many conflicts would be avoided if the character simply asked themselves- "What does being in love mean to me?"
And again, to be fair, this happens in real life as well- relationships that develop a little too quickly, and then implode just as quickly. The problem with this in fiction though is that rather than implode, I often see such relationship flourishing against all odds- and I can't help but feel like this is a dangerous precedent to set- especially when an adult writing YA fiction plays this as "what true love looks like"
Please, let's have more depictions of healthy, mature relationships- romantic or otherwise- that may be imperfect, but are nonetheless functional and mutually supportive. x')