The country seems more divided than I can ever remember in my lifetime. Not to mention breaking traditional rules (i.e. emoluments clause and releasing tax returns). Could create a bad precedent for future presidents. Also, weakening our alliances. The threat of nuclear war.
Perhaps it seems that way to you, but this didn't start with Trump. You could trace this all the back to the rise of the militant left in the late 60s, but I would peg the more recent breakdown in our civil discourse to Bubba's sordid behavior in the WH. This happens to be when alternatives to the big 3 news outlets really took off, particularly websites like Drudge Report, which broke several aspects of the Lewinsky affair that would have likely otherwise been buried.
This led to impeachment, which made the liberals super embarrassed and angry. And then GWB "stole" the election (he didn't) which made the liberals even angrier. And then 9/11 happened and the country rallied behind GWB, which made the liberals even angrier and frustrated. And then the Iraq war (which most Dems supported) went bad, and the Dems seized their opportunity. And then came Obama, who was way more liberal, partisan and divisive than any other president to date, pushing the country left on divisive social issues, which made the conservatives and blue collar people angry. And now comes Trump, who has not only sharply swung the pendulum back the other way, but won an absolutely gob smackingly shocking victory, breaking the brains of many liberals (I expound at length about this in the Trump Presidency thread).
So, this is been going on for some time, and put this divisiveness squarely on Trump is more than a little unfair. Liberals hysteria and radicalism bears most of the blame. After all, the very essence of "progressivism" is to promote a shift in society. You don't think that sort of change is divisive?