The biggest problem I see is his adamant, across-the board opposition to Civil Rights. This is a man who has faced court sanctions repeatedly for violations of the Civil Rights Act, only to buy his way out of them every time with empty promises. A man whose campaign was founded on the promise of discrimination against Mexians and Muslims, to the point that he was actually endorsed by American Nazis and the KKK. Hate crimes in the US have spiked along with his rise to prominence, and right now, anti-Muslim hate crimes are at their highest since 9/11.
My most immediate fear is that people will follow his example even more - already this morning, at a company employing thousands of people, I've heard a co-worker telling another co-worker about how the 'Dumb-o-crats won't be able to protect those criminals [any] more', and this was from an educated person in a business environment; the reaction from the actual Trump base - uneducated white people and older white people - is likely to be rather more intense.
We can also kiss the Affordable Care Act goodbye, along with its banning of yearly and lifetime payment caps, exclusions for pre-existing conditions, and outlawing of charging women higher rates because they're women. Based on GOP history to-date, I don't see them going any further than the repeal, either. They want it gone; they're not interested in restoring what few protections existed in what it replaced.
On top of that, there are all sorts of things a President can do to limit the First Amendment right to a free press (Bush II basically wrote the manual on that), he plans on breaking up immigrant families whose children are American citizens (and he has stated his opposition to the 14th Amendment's enactment of birthright citizenship), and his intention to ban Muslims from the US is a direct violation of the religion clause of the 1st Amendment.
It could also get interesting in that he currently has a fraud case going to trial this month, plus he is facing a potential 73 counts of fraud over Trump University, not to mention just dodging a trial for the ra** of a minor because of threats and abuse the woman has since received. Having a President-elect on trial for fraud when sworn in (and thus having the legal protections that go with the office kick in) could end up causing a constitutional crisis.
He will also be appointing at least one, and possibly up to four Supreme Court judges during his term, and has promised to stack it with judges willing to overturn Roe v Wade, which will return America to the days of coat-hanger abortions.
He has also repeatedly stated his objections to clean and renewable energy (in the case of windmills, because they might kill birds and because they cast shadows) and called for an increase in oil, natural gas, and coal mining. He also has stated for the record that climate change is a pro-Chinese conspiracy and that the intends to withdraw from the Paris Accords immediately.
Finally, not only has the man displayed an appalling lack of knowledge about how the US government works, but so far he has displayed the temperament of a particularly unruly toddler in both his personal and professional dealings. The man's solution to ISIS is to 'bomb them back to the stone age', and has shown every sign (to the point of asking about them in intelligence briefings) of being willing to use nukes as a first resort. I fear his reaction the first time someone stands up to him, or worse, badmouths him. The man is absolutely going to get us into another war.
Basically, I fully expect an America that is even more dedicated to the enrichment of the very, very few at the expense of the other 99.9% of the population, I rather expect to see civil rights in America set back 50 years, and I fear that we have a petulant child who not only controls America's nuclear weapons, but is eager to use them. And I fear that if nothing else, his approach to the climate will push it to the point that a runaway greenhouse effect becomes inevitable.
And I weep for idiocy of the people who believe him every time he lies about something that is easily proven, such as just last weekend when he claimed Obama was screaming and ranting at a protester. It's going to be a given that you should, under no circumstances, believe anything that man says.