There are policy issues of course. But one major problem is that he is a douchebag-- he speaks crudely and routinely demeans people- not just his enemies, but friends too.
He's also a an unprincipled self-centered narcissist.
A major problem is that he doesn't really have the knowledge, or the capacity to acquire knowledge that is required to be president.
A related problem is that in spite of his ignorance, he thinks he knows everything better than the people who actually do know stuff.