My problem with Trump isn't that he'll launch World War 3. My problem is that it'll take the country generations to overcome the damage he does. Since we're still getting over the GWB debacle, we can't afford another one just like it or worse.
So, something like massive tax cuts for the rich is an area where not only the GOP Congress and Trump will agree, they'll be trying to top each other in negotiations. The country can't afford that. I'm also guessing he can unilaterally ban Muslim immigration (but not round up legalized citizens - SCOTUS already slapped down Bush when he tried that). His anti science/anti vaccination bit is disturbing as well, especially when Ben freakin' Carson had to be the voice of reason in their debate over it.

Of course, there's the aforementioned Court picks.
Trump is the worst candidate I've seen in my lifetime.