Real world physics? People die falling out of a window or from being bitten by a poisonous spider or snake. They get paralyzed falling off a horse. They go into shock if they get shot. They die from a single knife wound. They break arms, legs, and ribs that put them out of action for WEEKS. And they break their necks falling down the stairs. It doesn't always happen, but for true "realism," all that has to be "possible." But if it were, most characters in an action-oriented game wouldn't make it through even 1 adventure.
Entirely true: a fall from a tree can result in ignoble death in a truly gritty system (such as the Real World). But flip that around: more interestingly, a fall from an airplane occasionally
doesn't result in death. In real life, people do survive buildings crumpled by earthquakes, or miraculously recover from terminal cancers, or get struck by lightning multiple times, or survive hails of bullets, or walk or of train wrecks with nary a scratch.
To my mind, these things are remarkable because life is, well, lethal.
Likewise in a game. There's little heroic if the hero get off scott free everytime something bad happens.
That's why I had followed up my original statement with the question about action points, or similar mechanics that provide a limited resource a player can use to counterbalance the inevitable, crushing defeats that comes with grittiness. I like things like hero points, death flags, fate, and related mecahnics because those sorts of devices allow (at least in principle) the PC to function in a truly dangerous environment, while reserving for the player a great deal of creative control over exactly how and when his PC can be the hero. Spending such a "fate point" (or whatever) sort of models those people who survive falling from airplanes in a world where just falling off a ladder can kill. In other words, despite the lethality of the system, the PC's wellbeing is still largely up to the player - just so long as he doesn't play the "get out of imminent horrible destruction" card
too much. I guess I just see it as a way to let the grimdarkers have their cake and eat it, too. Most of the time. Almost.
Anyway, that's just my 2cp worth of opinionizing.