Reading through the opposition reasons given here by Trump supporters reminds of answers to "what's your greatest weakness?" in interviews. I'll give some reasons against.
His stated policies are unnecessarily extreme. Securing the border is a good idea, but we don't need a giant wall to do it and we certainly don't need to destroy relations with our southern neighbor by forcing them to pay for it while rubbing their nose in it. We don't need to ban all Muslims from traveling to this country to keep us safe from Islamic terrorists. His tax plan will cost $1 trillion per year over the next ten years even factoring in economic growth from the tax cuts. He has said he won't make cuts to Social Security and he will increase spending on the military. You could eliminate the entire federal government outside of Social Security, Medicare/Medicaid, and Defense, and you would only save $750 billion per year. Trump's tax plan doesn't even come remotely close to adding up.
His core proposition that he will run the government like he has run his businesses is fundamentally flawed. You can't run a superpower like a business. The President is not equivalent to a CEO of a corporation. Trump has no experience in leading an organization this large where he doesn't have the final say on how things are run. The problem is fundamentally different. You also can't run foreign policy like negotiations between businesses. Money doesn't rule everything in foreign policy like it generally does in business. The prime example of this is his assertion that if he makes it financially advantageous for Mexico to pay for the wall then they'll do it. That is a preposterous notion. Foreign policy is as much about saving face as anything else. It would be political suicide for a Mexican politician to pay for the wall even if it cost the country more money to resist.
He doesn't have the temperament to be President. I'll just leave some quotes here and you can decide for yourself. Many Trump supporters cite stuff like this as a reason why they support him.
When Mexico sends its people, they’re not sending the best. They’re not sending you … they’re sending people that have lots of problems and they’re bringing those problems with us. They’re bringing drugs. They’re bring crime. They’re rapists. And some, I assume, are good people.
You could see there was blood coming out of [Megyn Kelly's] eyes. Blood coming out of her … wherever.
[John McCain] is not a war hero. He's a war hero because he was captured. I like people that weren't captured.
You know, it doesn't really matter what [the media] write as long as you've got a young and beautiful piece of ass.
Ariana Huffington is unattractive, both inside and out. I fully understand why her former husband left her for a man. He made a good decision.
26,000 unreported sexual assults [sic] in the military — only 238 convictions. What did these geniuses expect when they put men & women together?
[Angelina Jolie’s] been with so many guys she makes me look like a baby... And, I just don’t even find her attractive.
While @BetteMidler is an extremely unattractive woman, I refuse to say that because I always insist on being politically correct.
The concept of global warming was created by and for the Chinese in order to make US manufacturing non-competitive.