Least favorite plot device employed by way too many books you actually enjoyed otherwise.
For me, I think it’s probably the artificially-constrained superpowerful character. Like, say, Gandalf in The Lord of the Rings. Or Dumbledore in the Harry Potter books. Or Elric of Melniboné. It’s like the author wants to have a total badass but can’t figure out how to make the bad guy strong enough to create conflict. If the book is awesome, I don’t mind (and in some cases will actively argue that it’s okay) but when I look at it as a pure trope, it pisses me off. Like, a lot.