The same principle that allows a 20th level NPC off-screen to fall from a horse and break his neck logically also allows a dragon to kill him with a single bite, without whittling away at his hitpoints. It also allows him to die from an arrow through the eye fired by a Commoner 1. It also allows him to die by having a heart attack or choking on a fish bone. (Though certainly his 20th level cleric friend could bring him back.)
And all of these strike me as the DM basically ignoring the rules, and thus don't create a rewarding game for me.
Basically, adventuring- heck, being alive is always a dangerous business. Falling from a horse is an example, not the only point where the rules and the world don't agree.
In a game of heroic adventure, "being alive" isn't dangerous business for the heroes. It is for the commoners. It would violate D&D's assumed genre to kill Superman in a car accident.
If you want to make heroes a bit less invulnerable, there's a lot of cool ways to do that by the book, that would allow for 20th level fighters dying from falling off a horse, things that make the PC's more like Batman than like Superman, but it hurts my sense of verisimilitude when hit points get 'turned off' when the spotlight isn't on them.
Now I agree that in many ways, it is hard to make the 3.5 hitpoint model work for this. It strides an uncomfortable divide between hitpoints as physical ability to absorb damage and narrative protection. Again, there's a reason this is posted in the 4E forum. From everything we've heard, 4E will do a much better job of presenting hitpoints as narrative protection and determination/luck/skill rather than as physical damage.
Otherwise warlords wouldn't be able to heal people by giving them pep talks.
I think 4e will expressly state that hit points are luck, skill, endurance, and sheer cussedness. I don't think they'll mention narrative protection at all, because they
still won't go away when a PC isn't the narrative focus, because the 4e designers realize that not everyone enjoys this style of play.
I think in 4e, I will
still have trouble believing that a 20th level warlord can fall from a horse and die without resorting to DM fiat that, for me, goes too far.
I do think in 4e, I will be able to rationalize how a pep talk or a second wind can restore your ability to fight on. That doesn't mean that this idea of "rules are only there when the PC's are there" is in full force, but it probably means that the concept is given more attention than it was in 3e, which is a good thing by all counts.