Here's how I see it -- Trump completely unchecked would turn us into North Korea. The press would be turned over to the state, his kids would be appointed to succeed him, dissenters would be executed, the whole bit. Thankfully with our checks and balances, it would never approach that -- but we should be very alarmed by his nationalist and authoritarian rhetoric because presidents do have power to enact things via executive order. Clearly he would stretch that power to its absolute limits and be at odds with congress and the supreme court at every turn.
But the real danger is his sheer ignorance, hubris, stupid ideas and lack of any regard for how his administration would affect real people. A president does have lasting influence on foreign affairs and the economy, and the thought of Trump in charge of those things should scare the living #### out of anyone.