I think the main one is that he showed just how vulnerable our government is to partisanship. Expecting politicians to do the right thing according to tradition and convention, regardless of what their ignorant voters think? Nope. We might has well get rid of impeachment, because it's never going to happen no matter what the president does if his party has enough people to oppose it. I feel the Dems would be more likely to do so than the GOP, they tend to get rid of people caught in scandals (Weiner, Franken etc) while scandals just help GOP candidates win primaries.
Trump was a tireless champion for the ultra-rich, that poor downtrodden class. Tax reform alone was what, a nice 40% cut in their taxes (and for the low, low cost of trillions!)
Pesky environmental regulations were tossed out left and right (especially anything done by Obama). Who cares about mercury in the water, corporations need more money now!
He made us even more hated and distrusted around the world, which I guess to the GOP is a feature not a bug. Who needs 'em, you globalist...
He has set a new bar in how much you can just blatantly pull "information" straight from your ass with a straight face and have half the country think you are fantastic for it. We are sliding into an anti-science dark age full of conspiracy thinking and Trump is a major symptom of it. There are no facts anymore, just the ones you assert. The only good news is that I think the GOP is going to have a hard time finding someone as willing and able to spew dementia-sounding tortured-grammar ramblings that appeal to their base.