Ruin Explorer
Legend
This remains the biggest one for me, and it's particularly sad because it's completely an addressable problem, it's just D&D chooses not to address it - particularly not in 5E.I think the game world needs to acknowledge that high-level anything people can faceplant from the tallest tower in the land and just dust themselves off after. Similarly, that threatening someone with a dagger at their throat / crossbow at their face is actually really ineffective (especially because there is no condition for being helpless) - you need 10 friends with crossbows with you.
Death by extreme damage or whatever it was called in 3.XE largely solved the "I will just jump off this building and land on marble and be basically fine", but 5E abandoned that. A similar mechanic, even if it was only for falling damage, could solve the problem. Why doesn't D&D have it? I'd honestly like an answer from Crawford and co, because I don't think they have a good answer.
Dagger at the throat and out-of-combat stealth-assassin-type attacks are also really badly handled in D&D. Again they don't have to be - games with better mechanics, like Worlds Without Number handle stealth assassin stuff really well - that D&D both doesn't and doesn't care that it doesn't is another reason for caster dominance. Only casters get to do stuff like instantly one-shot or KO someone, especially with a single roll - or in some cases, no roll at all (hello Sleep!).
5E absolutely COULD have mechanics for this sort of thing, could make the fiction match the mechanics, at least in a "good enough for government work" way, but nope. Despite the fact that these are both fairly routine situations in D&D - far more common, I'd suggest, than actually say, sailing a ship or something.