An Army of Many
One side benefit of the flat math of 5th Edition D&D is than an army is actually a threat. Previously, the continually increasing math meant a small army of low level conscripts was about as useful against a party or high level adventures as swords made of damp cardboard. The king ends up with little authority over a party of murder hobos as even the threat of a few batallions of soldiers doesn’t phase them. This changes when the odds of a soldier hitting increases from one-in-twenty to one-in-ten while the adventurers might also miss twice as often. The army is suddenly a threat.
This also means that mortal kingdoms have a somewhat viable defence against planar invasion. A small tribe of natives isn’t going to do well against most fiends, but take, say, a company of bowmen…
On the other hand, the Tarrasque is as invincible as ever; pretty much immune to all non-magical damage, AC of 25, advantage on all saving throws against magic…thankfully it doesn’t regenerate any more. Unlike trolls…is something that recovers 10 HP a round and only permanently killable by fire or acid even worse?