1. She has a better temperament. The President's job is stressful, so I think temperament matters. Donald Trump seems excitable and impulsive, which in a President makes me nervous. Hillary Clinton seems like a calmer, more deliberate person.
2. Her base is more manageable. The alt-right is nasty enough as a fringe movement. If Trump wins, the alt-right will have a seat at the table in policy discussions.
Bad things would also happen if the Marxist left were to grow in power, but the situation isn't symmetrical. The relationship between Clinton and progressive activists isn't nearly as cozy as the relationship between Trump and the alt-right. And the Republicans in Congress won't roll over for the Marxist left the way they will for the alt-right.
3. She is more fact-based and can listen better. I don't like all of Hillary's policies, and I especially don't like her interventionist instincts. But I think she is capable of weighing the arguments of critics and capable of adjusting her plans in response to facts. Donald Trump can't take any criticism, and facts don't inform his plans at any stage.
4. Criticism of her seems overblown, while criticism of him is just. Matt Yglesias's take on the email thing is pretty much how I look at it, and for every bad trait of Hillary's, Trump has the same trait except worse (1, 2). The record of Trump's bad character spans decades. He is a small, insecure person with a cruel streak and a terrible lusting after power for its own sake. Lots of people are trying to explain how it happened—but however it happened, this year the Republican party nominated a candidate who deserves to lose.