Save or Die used arbitrarily is just as much a Deus Ex Machina as having Gandalf rise out of the floor to save the day. It doesn't add anything of value to the game if there are no points of contention to build up to it or lead away from it. It just seems gamist and mechanical at that point.
However, if a PC wants to wade through lava without protection magical or otherwise, I will let them know BEFORE they try it that they will surely die, and there will be no save. Some things need to be stated matter-of-fact to the players.
In a situation where a PC is on the edge of a lava filled ravine or a pit trap or some such, I actually like to use "Indiana Jones" models for this, and allow for a chance for rescue, because THAT can actually add a lot of new excitement and tension for the whole group of PCs to deal with:
<Ralgar failed his whatever check> Ralgar is scrabbling desperately for a solid hand hold at the edge of the lava-filled ravine as rocks fall away below him, he won't be able to hold on much longer.
I make it clear that Ralgar won't be able to get back on the ledge on his own. At that point other characters can attempt to assist him and the Ralgar character still has a fighting chance to survive or not. Those moments are the best times for a DM to get creative and build the excitement even higher. Ralgar may still fall over the ledge and perish in the lava below. Maybe none of his companions can reach him in time or they simply write him off, but at least as a DM I tried to build more excitement around the event instead of instant death for failing a single die roll.
As for monsters with Save or Die abilities... I often give clear warning signs of such a deadly adversary if they haven't encountered one before or they are taken by surprise.
Ralgar, the creature before you casts off its cloak, and you see a mass of writhing snakes atop its vaguely feminine head. <Ralgar fails his save versus whatever> Ralgar, the sword in your hand begins to turn to stone, and by mere luck you manage to drop it before the foul magic affected you as well. Your +2 Sword of Swording shatters in a rocky mess on the cavern floor.
Instead of instantly ending a PC for being in the wrong place and failing a saving through I wasted his favorite magic sword instead. Ralgar loses out, but at least the story can keep going. If the party chooses to stick around and try to fight this now obviously dangerous creature, so be it. And I warn them there is a real chance this creature will turn the PC's to stone. There are some uses of Deus Ex Machina like I have described above which can serve to build excitement and push the story to higher levels. Using Save or Die arbitrarily is on the opposite spectrum, but I would use it for the remainder of the battle if they decided to stick around. Some creatures are terrifyingly dangerous and should be. It's the players who should make the decision to embark down extremely dangerous paths, I shouldn't be doing it for them.
As for players having Save or Die abilities, maybe it should only affect creatures lower level than the spell or something. One shotting the BBEG doesn't add excitement or tension.
Save or Die can add to the intensity of the game, but I don't feel that it works "as is" for me as a DM. If 5e has SoD effects, I will simply continue to use them as I always have, but I would be really excited and happy if they can create some new interesting mechanic that works well with my own tastes so that you can have palpable lethality in a game without boring and cheap death.
