In my Dungeons & Dragons career I have been the DM of the group more often than I have been a player in a group. Nevertheless I do like to be a player from time to time, and I also think that it gives me a better perspective on the game and thus improves my DMing. Right now me and a group of friends are playing every Sunday on Roll20, in two different campaigns, with me and another DM alternating. Both campaigns started at level 1 and have just reached level 3 after two sessions.

In the campaign where I am a player, I chose to be a human fighter. Not very original, I know, but at that point the group didn't have a tank (in the second session a paladin joined us), and I think that the battle master fighter subclass is actually quite interesting to play. I made the fighter lawful neutral, and a noble, with the idea of playing him somewhat as a self-centered character. He goes out "to become a hero" more because of that is what his family expects from him than from any real desire to help somebody.

In the first adventure, at level 1, things start in a classic way. We come to a village, we hear that goblins have kidnapped the blacksmith's daughter, and we set out to kill the goblins and save the girl. There are combat encounters on the way to the goblin's lair, and the lair is a dungeon with several rooms, with more combat encounters. Some of them are quite tough, with a mix of goblins and bugbears. So at one point everybody in the group is low on health, spells, and other resources. Plus the bard, the only healer we had in the group before the paladin joined, had been knocked out and stabilized, but not revived, and needed to 1d4 hours of sleep before waking up again. So we left the goblin's lair and decided to take a long rest in the forest. While we did get attacked some more that night, we made it through the night, recovered our hit points, spells, and abilities, and finished the dungeon. Only that of course when we came to the final room of the dungeon, the blacksmith's daughter lay dead on the altar to the goblin god, having been sacrificed the night before.

Obviously on some level we "failed" in our mission. But it didn't really feel that way from a player's perspective. While the blacksmith was heartbroken, the other villagers were at least grateful that we got rid of the goblin menace. We still made enough xp to reach level 2. The treasure was the same. And on the roleplaying side, whether the girl lived or died didn't really make a difference to my self-centered noble fighter, as long as he could strut around as the goblin killer. As players, we didn't feel as if we had done anything wrong. Even if we had known, the other alternative, pushing on with your healer unconscious, still seemed a much worse idea than taking a rest. Already for out-of-game reasons of not wanting to play with one player at the (virtual) table being unable to act, but also because of the resource economy of Dungeons & Dragons 5th edition.

5E is fundamentally flawed, because at some point in the design process the developers imagined a typical "adventuring day" having 6 to 8 encounters, with 2 short rests in the middle, and a long rest at the end. And they balanced the different classes and their resources around that. 6 to 8 encounters is something like 20 rounds of combat. A wizard/sorcerer/cleric having 2 spells per day at level 1, and growing to something like 9 spells at level 5 is balanced if he recovers those spells only after 20 rounds of combat. It means that even at level 5, half of the time he is just casting cantrips, and he has to carefully judge when to use the big boy spells. A warlock with 2 spell slots from level 2 to level 10, who recovers those spell slots at every short rest, is balanced too. He'll have to use cantrips more often, but if he casts hex and has agonizing blast he'll be good. As will the fighter, who has even fewer special resources, like one action surge per short rest, but whose basic attacks are inherently more powerful than a wizard's cantrip.

The fundamental flaw in the design is that in practice that long adventuring day with 6 to 8 encounters and 2 short rests never happens. A part of this is because the system of hit points and healing doesn't support this amount of combat. Attacks in 5th edition are strong compared to healing. Even a perfect level 1 healing character, a cleric of the life domain, has his best healing spell heal just 1d8+3 points, while his best damage spell deals 4d6 points of damage. Even those lowly goblins deal 5 points of damage on average per hit; even if in every one of those 20 rounds of combat per day the enemies only ever land 1 hit per round, that is a damage pool of 100 points of damage, compared with maybe 50 hit points combined over all characters in the level 1 group, and maybe 15 points of healing if the cleric is willing to play the healbot.

But apart from the healing system not supporting 6 to 8 encounters per adventuring day, the class balance thing also quickly kills the idea. Those wizards/sorcerers/clerics with their best resources recovering at a long rest will not *want* to only use those only in less than half of the rounds of combat. Them, and because of that the group as a whole, will be much more powerful if the group takes long rests much more frequently. The idea of "short rests" is fundamentally flawed, because it is very easy to explain to a group that "you can't rest here", but it is very hard to explain why it would be possible to rest for 1 hour, but not for 8 hours. Basically the DM would have to invent story reasons that constantly counter the game system and the wishes of his players. That is neither practical nor a fun way to play. In the end, players care more about their characters than about the story. And so the blacksmith's daughter dies while the players take a long rest.

I could design an adventure that would work with the 5E adventuring day. For example I would use a dungeon and give the players a limited number of days to plunder it. After 5 days the king's army arrives and takes over, or something. So suddenly there would be some motivation to get more done in a day. But it is hard to see how you can do that in every single adventure you play. And if you play through published D&D adventures, this approach doesn't work at all. The Dragon of Icespire Peak adventure from the Essentials Kit (the new starter set of D&D) has lots of small maps with few combat encounters separated by days of travel. It isn't even possible to play a "standard adventuring day" of 6 to 8 encounters before a long rest with this published adventure, because there simply aren't that many encounters within a day's travel. And because the adventuring day isn't really used, the whole class balancing done around the concept isn't working.