Probably the fact that I write him to be not-omnipotent and not all powerful, though he has the power to overcome much of what he faces. I write using the Character's knowledge and experience. If they don't know something, then they don't get to use it. My MC has to earn his keep and train to achieve everything in front of him. He doesn't get handouts, which requires him to ask the stupid questions or read books to find his answer.
To some readers, it makes my MC look like an idiot. But if they actually put themselves in the MC's shoes, they'd realize he's just a regular dude who got tossed into a life he has no idea about and is honestly just trying to fake it till he makes it.
What some of them don't realize, likely because they skip over stuff, is that the MC does his due diligence before he throws himself into a furball. He's learned from his initial mistake and now takes the time, if he can, to properly read, learn, and prepare for the battles ahead. He utilizes the knowledge within The Library he has access to and takes the time to confer with individuals who may have more experience/knowledge of something than he does. It ruins some of the surprise, because I feed the MC proper information that someone would get if they actually asked a question to someone with knowledge (take a new chef asking an experienced one the proper recipe to a dish; they'd get the ingredients and likely be told how to properly cook and serve everything). But, if the MC doesn't ask, he doesn't get it. It's on him afterwards to do the labor and succeed.
So yea. My MC is a normal dude with some magic that he has no idea how to properly use and constantly deals with shit a lot bigger, badder, and spicier than he is familiar with and people don't like that I don't make him an OP Powerhouse.