I don't think he's fit to lead a country, let alone the world's superpower. I can go into specifics, I really can, but it would become a second career for me to do so. Therefore, let's keep it general. He's arrogant; he thinks he knows more than experts do. He's ignorant and he's not curious, which makes his ignorance incurable. He's ham-fisted in a world that is becoming increasingly more nuanced and requiring more finesse. He lies and makes shit up when it suits him. He doubles down when he's wrong. (If you trade, that should be a deal breaker for you right there.) He only "respects" those who admire and gush over him. He lashes out at any criticism as a matter of course. He's petty. He doesn't see the big picture because he doesn't take the time to think things through to get them right and consider the potential consequences of his decisions; he just acts (i.e., his immigration policy/Muslim ban). He doesn't know history and doesn't care that he doesn't. He talks mostly about himself wherever he goes because that's pretty much all he cares about.
I could go on, but why?