Swarmkeeper
Hero
I have yet to find a constant, fair way to threaten pcs within the framework of the game. 5e pcs are just extremely hard to kill without either flatly negating their abilities or throwing stuff at them they just don't have a chance against (like hugely overleveled enemies.)
I am defining 'challenge' as being dependent on player skill - just being less likely to win isn't more challenging. Requiring greater skill to win is more challenging. And in 5e, the amount of skill you need to master the game is piddly compared to many other games.
From the DM perspective, what difficulty knobs do you feel 5e lacks compared to past editions or compared to other games?