We've probably all seen claims by grognards, and even younger DMs, that 5th edition isn't as deadly as D&D was "back in the day". I think this is a statement that may seem true at first glance, but really begins to break down when you analyze it. If you're a DM, and you think this edition is too easy, or not deadly enough, make sure that you're aware of the following rules and permutations. It may be that it's the way you're running the game that's making it too easy, not the design of the game itself.
I also explore this topic on episode 145 of the Game Master's Journey podcast.
Bringing PCs Back From Death is Too Easy
One argument I often see for 5th edition being too easy is the claim that it's too easy for players to bring a PC back from the dead. Resurrection magic has always been a part of the game, so if we're looking for an aspect that's changed in 5th edition to make this "easier", I think the likeliest candidate is the spell revivify.
At first blush, it does seem like the spell revivify makes bringing PCs back from the dead easier than it has been in the past, and it does, with some limitations. Revivify is a 3rd level spell. This is a lower level "raise dead" spell than we've seen in previous editions. In the past, the lowest level spell that could bring back a dead PC was the raise dead spell, which was/is a 5th level spell.
A cleric receives access to 3rd level spells at 5th level. It's true, then, that PCs have access to the ability to come back from the dead at an earlier level than they did in previous editions of the game. However, revivify has some important limitations.
Only a cleric will have access to revivify at 5th level. The only other class that has revivify on their spell list is the paladin. However, since the paladin is only a half caster, a paladin won't be able to cast revivify until 9th level. So only a group with a cleric in the party will have access to this spell at 5th level.
Revivify must be cast within 1 minute of death. This means a couple things. First, the cleric must have the spell prepared, which means a cleric player must take up one of their prepared spells every day in order to be able to use it. Second, the cleric must either cast the spell during the combat or immediately after the combat. Casting revivify during combat could be problematic to say the least, as the revived PC will only have 1 hit point until healed. The likelihood of the revived PC being sent back to 0 hp again before the end of the combat is pretty high. This is also a round that the cleric PC can't do much else of use, like healing someone, making an attack, or casting a spell. However, waiting until the end of combat has its own risks. If the combat takes longer than 10 rounds (1 minute--which isn't likely, but could happen), then it may be too late for revivify, which must be cast within 10 rounds of the PC's death. This also means the cleric must survive the combat, and might even need to be conscious. Bringing a downed cleric back to consciousness will take a round, and the clock is ticking with revivify.
Revivify is expensive and uses a hard-to-acquire component. Each casting of revivify consumes diamonds worth at least 300 gp. If you're keeping within the parameters recommended in the DMG when it comes to the encounters you throw at your party and the treasure rewarded by those encounters, 300 gp will be a substantial cost for a party of 5th level PCs. It's enough of a cost that they may debate the investment for just one set of 300 gp diamonds, and it's fairly unlikely they'll have more than one, or many more than one. Finding such valuable gems would only be possible in a large city, and even there it would not be guaranteed. The PCs might need to journey to multiple cities before they find enough, or they might have to wait for a jeweler to procure them. Once the diamonds are found, they may cost more than 300 gp.
When we consider the other spells in the game that can bring fallen PCs back to life, there doesn't seem to be much difference from earlier editions. Raise dead is a 5th level spell, which means a full caster won't have access until 9th level. Bards, clerics and paladins all get access to raise dead, but the paladin won't be able to cast it until 17th level. Raise dead can be cast within 10 days of a character's death, so a PC doesn't have to prepare it every day. It does take an hour to cast, though, so using it in combat won't be an option. Like all the resurrection-type spells, raise dead has a costly and rare component, in this case a diamond worth at least 500 gp. This may be even more difficult that the revivify component because it has to be a single diamond worth at least 500 gp (instead of a collection), which won't be easy to find.
I make the same arguments here that I did for revivify above. 500 gp isn't (or should not be) a trivial cost for PCs of 9th level, and a 500 gp diamond isn't (or shouldn't be) easy to find. Finding multiple 500 gp diamonds would be even more difficult.
Raise dead also has the limitation that the character raised has a -4 penalty to attack rolls, saving throws (including death saving throws) and ability checks (this is pretty much every time you're rolling a d20 in the game). This penalty is reduced by 1 each time the character takes a long rest. This basically means that the PCs will want to take 4 days of downtime after raising a character from the dead. If this isn't possible, things can go badly for the PCs. For example, using raise dead in the middle of a dungeon could easily result in the same PC dying again in the next encounter, as a -4 to death saving throws is pretty significant.
Druids get access to a similar, but worse, spell at 9th level, reincarnate. This spell has the added drawback that the PC comes back as a random race. It also costs 1,000 gp to cast (in the form of rare oils and unguents that should be at least as difficult to find as high-priced diamonds) instead of 500.
Resurrection is a 7th level spell that bards and clerics will get access to at 17th level. Like raise dead, resurrection has a casting time of 1 hour and gives the -4 penalty to attack rolls, ability checks and saving throws. Resurrection burns up a diamond worth at least 1,000 gp.
I don't think bringing PCs back from the dead is too easy in a 5th edition game where the DM isn't taking it easy on the players. If the DM is taking it easy on the players, then the criticism of "too easy" should be leveled at the DM, not the game. Here are some ways that DMs make bringing PCs back from the dead too easy:
Giving out too much treasure, diminishing the limiting factor of the expensive components
Using encounters that are too high of a challenge (CR) and then softballing them (resulting in too much treasure), diminishing the limiting factor of the expensive components
Making the expensive components too easy to get, making it too easy to stockpile these components, or even ignoring the cost of the components entirely
Giving the PCs enough downtime after bringing a dead PC back that the penalties from resurrection and raise dead become meaningless
Letting PCs run around carrying gems worth thousands of gold pieces without ever getting robbed, having to leave equipment behind, etc.
If you want death to be a real penalty in your game, and you don't want the use of resurrection-type magics to be too easy, all you need to do is run the game as presented in the PHB and DMG.
Killing PCs is Too Hard
I've found 5e quite deadly, more so than 2nd & 3rd, which are the other editions I have the most experience with. Getting a PC to 0 hp might be a bit of an issue at higher levels, but once there, death is difficult to avoid.
I think one reason some DMs may feel the game is too easy is that they haven't completely internalized all the mechanics that come into play when a character is at 0 hp and making death saving throws. They also may be softballing encounters, being too nice to the players, and not playing intelligent adversaries intelligently.
The following rules come into play when a PC is at 0 hp (or anytime a PC is unconscious, see the unconscious condition in Appendix A of the PHB):
Any damage taken at 0 hp results in a failed death save. This includes damage from area effect spells (e.g. fireball), environmental damage, falling damage, etc. Any decent high level encounter should have enemies using AoE abilities or environmental damage, if not both, so that's at least one automatic death save failure per round for downed PCs, if not more, in many encounters.
Unconscious characters automatically fail dexterity and strength saving throws, so they will be taking full damage from those AoE spells, like fireball.
A smart NPC will finish off downed PCs if they know the party has healing ability. Any attack against an unconscious character has advantage. Any damage taken by an unconscious character results in an automatic death save failure. Any attack within made against an unconscious character by an adversary within 5 feet of that character that hits is an automatic critical hit. Critical hits result in two automatic death save failures. NOTE: Assuming that an unconscious PC is prone, any attacks made against that PC from an opponent more than 5 feet away has disadvantage, which would cancel out the advantage of attacks made against unconscious characters, making these regular attacks (see the prone condition)
If a PC at 0 hp takes damage equal to his hit point total, he dies instantly. This is less likely at higher levels, but could happen with a critical hit, which any hit from an adversary within 5' will be. Instant death by critical hit can happen very easily at lower levels.
A roll of a natural 1 on a death saving throw results in two death save failures. There is a 5% chance of this happening every time a player rolls a death save.
Three death saving throw failures equals death.
For these reasons, even at high levels, an encounter with intelligent NPCs or monsters that the DM isn't softballing will be very deadly to any PCs that get to 0 hp. Oftentimes, PCs at 0 hp could be killed before they even got a chance to roll their first death saving throw, unless the next PC in the initiative order is able to heal them.
If an intelligent NPC or monster knows the PCs have healing magic, they will most likely finish off PCs that are dropped if they're truly trying to win or survive. Most times it will only take a single action or attack to do so, and a wizard who throws a fireball when there are multiple PCs at 0 hp can really wreck a party's day.
NOTE: In most cases, the difference between an unconscious and a dead character shouldn't be obvious during a combat without taking an action to do a medicine check. In fairness, NPCs that use this tactic should make an additional attack against all downed PCs, regardless of whether the PC is dead or at 0 hp, because the NPC won't have any way to know and isn't going to waste an action to find out. Of course, this rule should apply to PCs as well, and players who assume they know if a downed PC is unconscious or dead (or how many death save failures and successes they have) are metagaming.
If you're someone who believes 5th edition is too easy, ask yourself if you're really running adversaries as cunning opponents who want to survive and win just as much as the PCs do.
For DMs (or players) who believe NPCs wouldn't really use these tactics, I propose the following experiment: Start using the death saving throw rules for all your monsters and NPCs, just as you do for PCs (this is an option suggested in the PHB). After a few encounters of this, once the PCs realize that any downed NPC could be brought back into the fight by a healer (instead of just being dead at 0 hp), see if and how their tactics change when they're fighting NPCs or monsters that they know (or even just suspect) have access to healing, whether in the form of spells, magic items, or potions. I'd be willing to bet you see them begin to attack downed NPCs, no matter what their alignment is.