His lack of humility.
Trump obviously thinks he is hot shit. I, I, I, I permeates his speech. He tells us that he is very rich every chance he gets. I have never met a rich person who actually tells you he is rich. It just does not happen. Only wannabes have to resort to boasting.
Trump also often says things like “Only I can fix this and that.” That’s another indicator of low self-esteem or overblown ego, both of which are not acceptable in a president in my estimation. There are a thousand people in the country how can “fix” it who are eminently more qualified than Trump. They are just not running for president, or they didn’t make it past the circus of the primaries.
Humility Trump does not possess. But our president needs inherent humility to be successful.