In these three posts, we have the entire resumé of this entire thread.
Yes people and a lot of them complain that 5ed is not deadly enough. This prompted many different solutions from them. Some are more or less a simple patch, others makes the game quite deadly for no reasons (IMHO). Others, the majority, are stuck with their perception and stick to rules.
I must've helped about 30 to 40 DM in making their games harder without resorting to too much house rules. Each time I have seen a combination of factors.
1) Not respecting the 6-8 encounters per day. Because of many reasons but mainly story (go figure) and pacing (go figure).
To these, I often refer to the rule
@Oofta uses (short rest a day, long rest a week). It clears their problem.
2) They make the availability of rest soooo easy. All PC do are Novas...
This is also known as the 5 min work day. Again, I refer to the 6-8 encounters per day. But I also tell them to only allow a maximum of two short rests per day and a maximum of 1 long rest per day. Once done, if the players are not interested in adventuring further because the last nova drained them too much. Send as much encounters it will take to bring them to the 6-8 encounters. Rests can be interupted. A Tiny Hut can be dispeled or its position tracked downed (with skill or magic). Do not be above to add a shaman or an NPC humanoid able to track or both. Nothing is more stressful than to see a few encounters worth of humanoids massing around the hut and seeing them with a post sign: "We knaw ya're in there. Our shamaaan may not be able to dissipitate but his magick told him tha ya were thare! Come out adventurees!" My players did learn their lesson the hard way...
Second syndrome related to rest too easy. Monsters are static and wait for the PCs...
Yep, a lot of inexperienced DM will allow forays into the Dungeon and the players are safe when they leave. Have the inhabitants make seek and destroy teams to find the PCs. Have the kobolds prepare for the next foray. And, if their ploy fails, have them simply leave the area with all their treasures. That is hurting the players more than losing their characters.
3) The whack a mole syndrome.
Again, too lenient DM allows this tactics without doing anything about it. But there are no negatives! I often hear. So what? Intellingent monsters will first put down the one that was just upped. Make enough attacks so that he dies for sure (as in the character is at zero, so let's hit him three times to get over with his case once and for all). Then, they will concentrate on the damn healer.
Even non intelligent can make the whack a mole a bad thing. A wyvern is a predator. The character falls? GOOD! A GOOD MEAL! And whooppsies... wyvern wins the next initiative gobbles the PC and flies away...
As
@Crimson Longinus said, D&D is about action heroes. I firmly believe that (I know, I tried other grittier systems) and I like D&D to be this way. But at the same time, healing overnight is a wee bit to extreme in the action hero genre for my tastes. That is why I do not allow healing overnight. You have to use your HD. Just that was enough for me. Other use rest variant, other use gritty realism. To each his own. But there are solutions that are not as drastic as crit and lingering injuries tables. Again, to each his own. But D&D can be deadly if you play by the rules, make the monster vivid and play them at their intelligence level. The real question is not if D&D is deadly, it is how deadly do you want it to be?
PS: The second question should be: "Why did the authors did not tell about these optional rules in the PHB?The rest variants, the grittty realism, the necessity of 6-8 encounters and no healing on long rest are easy enough to find in the DMG and do not take that much space...