Special magical important bloodlines. "He's inherited that secret hidden amazing technique from his father, no training necessary"
Multiply that irritation by a factor of fifty if the "common" MC discovers that they actually have superior, unassailable powers because they are secretly related to royalty. An "ordinary" girl discovering that "she is actually super-duper special because her grandmother was actually the witch queen and no one knew about it till now, so now the bullies have to beg for forgiveness" makes me want to toss the book (or my phone, if reading digitally) into the nearest open flame.
Runner up is killing or maiming a more experienced, more competent, more useful MC so we can instead follow the clutzy inexperienced "real" MC that the author clearly wanted write instead.
Why taunt me with the cool, dramatic, engaging story that could have been just to swerve towards Palm-Sweat Knock-Knees over there and watch him fumble his way through character growth and then fall ass backwards into beating the big bad?