PC durability is absolutely my biggest complaint about D&D5 (it's still not a very big complaint, though). Beyond about 3rd level, barring very specific abilities and spells that don't make sense in all campaigns or all encounters, a TPK is more or less the only way to put a PC down by the RAW.
But I don't think the real problem is that PCs are hard to kill. The problem I'm seeing is that a player with a character at 5 HP plays his character the same as he would at full health, because the risk to him is so minimal. With a life cleric in the party being knocked unconscious is barely an inconvenience.
Which is why I're revised my position on coup de grace. I've always thought of it as a dick move, and in previous editions where a single attack would kill outright I think it was. PCs were unlikely to get back up during combat, so the coup de grace was reserved for only the most villainous of opponents with an interest in demoralizing their foes. Using it more often smacked of desperation on the part of the dungeon master -- one should not have to resort to mean-spirited tricks to challenge one's players.
But in a world where a fallen combatant is decidedly /not/ out of the action, monsters are going to learn to make sure they stay down. Smacking a downed enemy once to rid it of two death saves before moving on is a very attractive strategic consideration. And it means PCs are going to be a lot more careful about being at low HP.