It's not only the lying, which is bad enough, it's that the people that should be holding him accountable don't.
The lies just flow into the bloodstream, unchecked, unquestioned and morph into truth. Reality isn't reality.
The fact that it is now seemingly normal for a world leader to just make shit up without consequence is really, really alarming. It doesn't help that this is amplified by what is essentially a state propaganda arm in the form of Fox News.
I now understand how fragile our liberal democracies are, and it terrifies me.
For a laundry list of the outright lies, half truths and false statements check out all 31 pages of Trumps Politifact page.