Let's say that there are some occasions where the DM sets the rules to one side and rules by fiat that the PC dies.
"Rocks fall, everyone dies" is a famous example of a DM acting UN-fairly. If the DM is ruling by fiat and deliberately ignoring already existing rules, this is an occasion where he will be judged by his players as to whether he is being fair or not.
Guillotines don't kill by reducing hit points. No damage is rolled. They kill by removing your head. Therefore, if one removes your head and the DM rules that you are dead, is he being fair?
Most of us you say yes, that's fair. But what if you are a creature which in game does not die if its head is removed, like an elemental or a golem? What if the DM ruled by fiat that you died anyway? Would that be fair?
Dragons, the most ancient and powerful dragons, are very dangerous. Let's face it, a gargantuan magical genius with all the dragon stuff, both in concept an in game terms, should easily kill a mere human. Let's say your 1st level fighter charges the dragon. Yeah, the DM could use the already existing rules, but what if the DM rules by fiat that you die. You might think that it's fair enough. But what if your epic barbarian with his Boots of Flying, Dragon-Slaying weapon, Armour of Immunity to Dragon Breath and the epic Magic Resistance ability? Would it be fair if the DM decided by fiat that your barbarian was dead without bothering with the rules?
I think that fewer of us would think that was fair. Further, I think that most of us, thinking of a spectrum of PCs fighting this dragon, from 1st level to epic, would believe that it is more fair to decide by fiat that the PC dies the further the PC is toward the 1st level end of this spectrum, and less fair to arbitrarily decide that higher level PCs die by DM fiat.
That seems reasonable. The less likely the PC to survive, the more likely to decide by fiat that they die, and the more likely they are to survive by the game rules, the less fair it would be to kill them by fiat.
Now look at the falling rules, and a fall of over 200 feet. We know that the rules say that this does 20d6 damage, and we know it's much more likely to kill a 21hp PC than a 119hp PC.
But the DM could rule by fiat. The lower level ones would be more likely to die, right? Just like the dragon example.
But the impression I get from this thread is that there are those who would be happy to use the rules instead of fiat versus the 21hp PC, the ones for whom we could forgive death by fiat, and eager to rule death by fiat for those who they know the rules mean that the PC is overwhelmingly likely to survive!
This is on its face unfair. The DM isn't taking a shortcut to the same conclusion as the rules, he is ruling opposite the conclusion that the rules indicate!
Two pupils are taking a hard exam. The pass mark is 80%. Thicky McShort-Plank is unlikely to pass, while Captain Brilliant is likely to pass with flying colours.
Instead of having them actually take the exam, the teacher, Mr. D. Master, has the power to choose one boy to go straight to Harvard and the other to Burger-Flipping school. Most of us would think the fair decision is that Captain Brilliant goes to Harvard.
But I'm arguing in this thread against those who would send Thicky McShort-Plank. Deciding that a PC most likely to survive taking 20d6 falling damage is the one that more deserves death by fiat is not fair or consistent, and this is contrary to the behaviour we expect from a fair and consistent DM.