The real question would be why so many lower to middle class people voted for Trump. One of his main policies, and one that really makes him a Republican, no matter how much he claims to be different from the rest, is going to cut taxes for the super-rich and cut social benefits for the poor(er). Once again the 0.1% managed to get their candidate elected by the people who understand the least about politics and react the most to rhetoric, populist demagogy and all the media drama. And the sad thing is that all these people are happy that their candidate won, not realising that they voted for the 0.1% (all the opposite of what Trump appeared to be, despite his being part of the 0.1%). It's just like with Brexit all over again. Those who voted for it will be the ones suffering most the consequences. There is no cure for idiocy yet.
On the international scene, I was very concerned when G.W. Bush got elected, and he indeed caused a lot of wars and suffering (Afghanistan, Iraq) as was expected. Trump could be much worse because his attitude is even more radical. I disliked Bush for being an evangelist Christian, and Trump is better on that front as he doesn't seem to care much about religion. Yet, his extreme stance against the Mexicans and the whole Muslim world will cause serious tensions globally and will certainly foster more Islamic terrorism in retaliation. On the other hand one could wonder if it wasn't about time Islamists like those ISIS bast*rds get crushed once and for all.
If Trump leads the country properly in the next six months, or at least satisfy the masses by implementing some popular campaign promises, his example could bolster Le Pen's chances of becoming France's next president in May 2017. With both France and the US led by anti-Islamic presidents, a domino effect could see a rise in radicalism across Europe, which could very well degenerate in WWIII (the West vs the Islamic world). Who knows? Maybe we are already in it and don't have enough hindsight to realise it.
Trump's victory is not all bad. He is strongly against the TTIP, which I think was a huge mistake - at least for European consumers, who would lose the benefit of stricter food regulations.