Sparse comments...
I think the issue is just a matter of CR calculation and counting action economy of enemies (XP multiplier) when players already have area spells and the like.
Maybe one reason why 5e is often perceived as "easy" could be that they were quite conservative with XP budgets. In addition, many DMs probably don't scale the encounters up when they have less than the suggested 6-8 per day. But I also think that MOST DMs definitely do not play monsters tactically as well as how the players play their PCs, and then just throw more badly-played monsters and then wonder why the PCs are levelling up so fast.
Actually, traps are a good example. Let me ask you this. Is the Tomb of Horrors challenging? Is it equally challenging if you know the answers to all the traps?
Do the wrong thing, die. Do the right thing, move on. The problem is, after you solve it, it is boring. Because you just repeat the same steps.
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But "poison kills you if you fail a save" doesn't give the player a choice. What choice is there? Never get ambushed by a spider? You don't want to get ambushed anyways, so you are already trying to prevent that. Don't fail a save? You don't have a choice in that matter.
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Thank all the dice gods that Vancian is nearly dead. I hate the idea of trying to predict how many times I need a spell.
I think traps are indeed a good example about how the rules are not what provides the challenge. Rules that reduce a trap to one or more dice roll remove the challenge from the player. I don't think there can be a ruleset that really makes traps as interesting and challenging as treating them "rulelessly" one by one.
Poison is one of those things that GAMERS demanded for years to be dumbed down to a mere extra damage bonus, so there is little surprise that now it's boring. You have to look for non-standard and narrative-driven poison to introduce possible choices. Once again, a ruleset just gets in the way.
With Vancian I disagree. Having to choose each slot WAS a challenge. But it was a challenge YOU (and many others) didn't enjoy. I was fine with that, but I am just as fine with 5e compensating with a smaller amount of prepared spells per day. What I only want to say is that in every ruleset or edition there are challenges that some people like and some hate. The designers remove challenges which they think they have more haters than lovers.
Death saving throws make it easier to kill characters for me.
I am still undecided on this, but I think I probably agree.
With 55% success on each DST, roughly you get 4 rounds on average to be saved by an ally. IIRC in 3e you lost 1 hp per round when negative, and died at -10, so maybe the average number of rounds to be saved was slightly longer.
But there are other things that matter, e.g. how easy it is to stabilise an ally, and how monsters can affect a dying PC.
I think the biggest issue I have with this thread is the suggestion that DMs just attack downed characters.
This doesn't sit right with me. Sure it makes the game more lethal but there's an element of antagonism here.
I never do that either. I prefer to default to a creature to move on to the remaining active threats rather than spending its actions on a currently disabled threat.
IMO, 4e has the most challenging battle system. It is the only version of D&D I have played that becomes significantly more challenging at high levels, not significantly less challenging.
I haven't really played 4e, but all editions I've played were MORE challenging at higher levels, because there were more possible kinds of threats/effects to face and a larger number of resources to manage.
There were also more possible "winning buttons" of course, but more likely to trivialize parts outside combat, such as travel, investigation or food. But with the exception of investigation which is a big deal to lose, other stuff was considered a nuisance by many groups who maybe expect more heroic deeds than worry about travel at high level.