Blood of Prokopius blogged the other day about the lethality of OD&D vs. that of the newer editions. While his supposition that this is a reflection of our culture's general discomfort with death is interesting, I'm not sure that's the main reason. I think a lot of it has to do with how much we put into new characters.
In OD&D, one can make a new character in fifteen minutes flat--maybe thirty if your group hasn't thought to put together "adventuring kits"--equipment bundles of the common stuff you need to survive a dungeon. In editions 3+, it can take 30 minutes to an hour for even an experienced player to properly tweak a character's race, class, skills, modifiers to skills, skill synergies, feats, weapon and armor stats, spells, equipment, etc. Moreover, today's gaming culture tends to assume that you should have a history and personality that goes along with those stats. It's not just the time, it's the personal investment of imagination that we put into a character, especially when the rules state outright that the character is already a hero by the time he or she enters the story.
So naturally, the DM will feel a bit more inclined to spare that newly-minted 1st level character from that lucky arrow strike in the first round of combat that would have killed him outright in OD&D, whether with hp padding, generous death & dying rules, or just fudging the roll. That, or we can expect the character's identical twin to show up thirty seconds later.
This is one reason why I prefer to randomly determine a character's background (aside from messing with the munchkins); it speeds up the creation process and removes a bit of the investment from a poor mook who might not make it past the first room of the first dungeon.
Of course, there should be investment and depth in a character, but I think these should develop over time as the character survives and (maybe) thrives in the campaign world, proving by his longevity that he's not just a redshirt to other, "more important" characters. Everyone wants to assume that his character is Aragorn--but what if he's Boromir, or just one of the many common soldiers who perished in the siege of Minas Tirith instead?
The proof of the pudding for a character in OD&D is not having a heroic lineage, making him the last remaining heir to the throne of a fallen kingdom, or being a mighty-thewed barbarian. Boromir was the heir to a kingdom (though as Steward rather than King) while King Arthur was a servant boy and more heroes than one can count came from humble farmer or herder backgrounds (Rand Al'Thor, Paksenarion, and Samwise Gamgee, to name a few).
To paraphrase Batman, it's not who a character is inside that matters, it's what he does that defines who he is.
So then, let us make use of the referee's favorite tool for handling situations where the players just won't let go of that red herring: Schrodinger's Gun. In the player's case, this means that until he has actually voiced or written down an aspect of his character's background, it's in a state of indeterminency.
For example, a character's background may indicate that he was a professional soldier before losing his marbles and deciding to hunt goblins underground, but it doesn't say in whose employ. Now say that a particular adventure requires getting access to a certain baron. "Hey," the player can suddenly chime in, "I was an archer for him back in the day. I left on good terms." If this seems just too terribly convenient for the referee, they can always roll for it, but in general the player's statement should be allowed to stand.
Of course, that means they also inherit their lord's enemies. So what if the company ran across a group of them before, but not knowing the character's (at that point non-existent) background, the referee ruled (after a reaction roll) that they reacted in friendly fashion. That in itself leads to an interesting development. Perhaps they didn't recognize the character. Perhaps they did, but are the sort who treat a worthy opponent with respect. Perhaps they feigned ignorance or respect and trailed the party to see what they were up to, and now the character's de facto enemy is informed as to their whereabouts.
The same is true of skills that come out of backgrounds. The player rolls the herder background for his character. Later, when the party is trying to make their way down a steep cliff, he argues convincingly that his character herded goats in the highlands, and should be counted as skilled in climbing and balance. He could not later claim to have herded sheep in the steppes when the party is stuck in a vast plain looking for food and water.
To use this system, record-keeping, both by the players and the referee, is key. As each new element of the character's background comes out, it must be written down along with the skill level or other bonus that it grants, just as the character's ongoing development--like when and how he trained various skills--must be.
In the end, if the character meets a swift doom, the player won't feel as cheated for having blown an incredible background on him, but if he survives and thrives for a time, he will grow into a well-developed, three-dimensional character who will be remembered fondly.