Lava being instant death but many other common sense 'fatal' things not being instant death is bad design.
Agreed. 100%.
Except...
You may as well make falling into a spiked pit instant death
Why...? Falling into a spiked pit
isn't necessarily instant death. The save for it means that the character might have grabbed onto the lip of the pit, or was able to get their feet to land between the spikes. A low damage result from a fail could mean they were only grazed by a couple of spikes...painful but not fatal. Or the effort of twisting to avoid a fatal fall results in straining some muscles, etc.
There is common sense involved in a spike trap not being instantly fatal.
...or all critical hits instant death.
Again...Why?!? Critical Hits can be as definitive as instant death (if taken to zero HP or lower), or simply a significant but
non-fatal wound.
Not all Critical Hits are Instant Death...some just leave you really messed up and you're never the same again
...but not necessarily unable to continue a fight.
Now, getting caught in a trap that involves a 2,000 lb. block of granite dropped on top of you in a pit, and surviving...
Or falling off a 200' cliff, and surviving...
That's Bad Design.
You should need a lot better protection than a second level Resistance to Fire spell to survive immersion in lava, but there's no reason why it shouldn't do a large but possibly survivable (with lucky rolls or a serious investment in healing or fire resistance on an already tough character) amount of damage. Making near fatal hazards tends to lead to more interesting games than just having the consequences of every other mistake be "you die". Look at Magic The Gathering, most cards that cost 7 or more mana, 95% of the time, could just say "You win the game.", and the ones that don't aren't worth using, but the 5% of the time when you somehow manage to pull it back leads to the most interesting stories.
It also works both ways. If lava isn't instant death, you can use it as a hazard in boss fights without risking players killing said boss in a single hit with a repositioning move. Successfully knocking a boss into lava should drain a good chunk of its HP, but if you're at the level where greater demons and dragons start showing up, they should be able to keep going.
I agree that being able to survive a fall into lava should take much more than a low level spell. But how is it common sense if one fails their save, falls into lava, and then simply crawls out and continues fighting (excepting some significant protection against it)?
Falling into lava has only one of two possible outcomes:
Death or
Massive Maiming and Completely-Debilitating Injury. Both of which leaves a character/monster/NPC unable to continue
any action...especially fighting.
I could see a rationale for making lava reduce a victim immediately to 0 HP without immediate death. But simply losing some Hit Points (or a lot of Hit Points), then climbing out of the lava and continuing to fight... That doesn't just stretch common sense, it utterly destroys it.
So...
Falling in Lava = Instant Death
or this...
I think I might actually prefer Instant Death...