Policies, starting with his hiring practices, managerial style, but his open baiting of anybody to his left didn't help, nor his lack of ethics and morals, didn't help, along with his big mouth. Though I didn't vote for him (on evaluations of morals and ethics and lack of experience), I was determined to give him a chance, but by 9 months in, I knew I was correct and his brand of party populism to entertain his supporters was going to be destruct in the long run, to himself and the country as a whole. You really can't hire people that are crooks and under indictment or suspicion before you even came into office, without it turning into a sideshow of embarrassing legal entanglements and dismissals, resignations, etc. It appeared several in his cabinet were hired specifically for their lack of experience in their appointed position, but to more or less muck up the agencies, he didn't like, such as education, getting somebody who was not ever an educator at any level, Dept of Interior and EPA from the mining and oil industry, etc. His thing of "We're going to have only the best people" was a sham, from the beginning, as seen by his revolving door personnel management, of executive positions and advisors. By September, I gave up on him growing into the gravitas of the position.
You could say, he screwed up immediately on the same things that would not fly for a public sector CEO, that had to answer to a board, not controlled by his ownship and stock holders/stake holders.