I've been pondering PC death a bit more than usual recently, as a consequence of reading various Old School narratives at Beyond the Black Gate and The Alexandrian, as well as the resurrection of the EnWorld 2011 PC Bodycount thread. I suppose Fjolkir's deathwish and Somak's sacrifice might be helping, too. Basically, it seems to me that there are two reasons PC deaths occur in RPGs. The first is unexpected death as a consequence of poor planning or poor luck; the first-level monk charges the grizzly bear alone, misses with his one shot, and then is mauled (true story), or the first-level fighter eats a crit from an orc with a greataxe (also true story - back in 3.0, orcs came with greataxes for d12 damage and x3 crits. They changed them to falchions in 3.5), or a lightly-armored Traveller character decides to provoke a skilled opponent with heavy weaponry. This seems to be the most common kind of PC death - it's what BtBG and the Alexandrian are talking about. Sometimes you just suddenly die, and it kinda sucks. You get stuck building a new character, and the DM gets stuck reworking any plotlines that you're (er, were) critical to, or you have to get resurrected and lose a level if resurrection is available to you (speaking for 3.X here; don't know if older or newer versions handle resurrection differently). In any case, it's not a particularly fun experience, and you probably feel wronged by the DM, the dice, or the rest of the party for not telling you not to charge the grizzly bear (unless they did and you ignored them, in which case you feel sheepish).
The other kind of death, though, can be fun. It's the heroic sacrifice, the martyrdom, the death by player choice. When you, as a player, go "Yeah, my character is willing to die to achieve this objective", and then you make it happen. From your perspective, you've taken the spotlight of the scene and hopefully seriously influenced the outcome; you get to be the Big Damn Hero. From the rest of the players' perspective, you've gone and done something they probably weren't willing to do; props to you (plus, hey, now they get your loot). From the DM's perspective as storyteller, the act is useful because the sacrifice trope has a strong emotional impact; you've gone and given him something to work with. There's no resentment, no DM-Player antagonism going on. It's a win all around, except for the character. Prime example from experience here is Somak's sacrifice, but there were one or two examples from the Bodycount thread of the Sacrifice as well.
The sacrifice death is, essentially, a narrativist conception of death - you die for the story, or as a final act of characterization, by your own choice. Consequential death, likewise, is essentially a gamist notion - you die as an outcome of the game. Finally, a blend of the two is relatively simulationist of a realistic world, where sometimes people die to further a cause, but usually people die as a result of external factors. I think this recognition of the causes of PC death as linked to the GNS paradigm could be useful if you're striving to embody a particular part of that paradigm; if you're trying to run a game where the narrative is king, you could remove death as a likely consequence of combat and make it purely by player choice, much as E6's Death Flag rules do. Likewise, in a strongly gamist game, there might be no benefit to be gained by the act of self-sacrifice; death is losing, and if you lose you can't win. I would argue that 3.X does a pretty good job of this as far as mechanics go; with a few possible exceptions (epic spell backlash and that one Wu Jen spell, for example), there are almost no rules that let you die in order to achieve a grand effect on a mechanical level.
Pursuing a more narrativist perspective on PC death has interesting implications, though. In a game where you can't die except by choice, combats have to have a little more meaning than "kill or be killed" - there has to be something else at stake. On the flip side of that, though, if death isn't the result of hit point / resource depletion, what is? There needs to be some penalty for running out of stamina. Further, how best to allow the choice to die to generate mechanical benefits while being neither overwhelming nor ineffective? There's some design space to be explored here for a "death for effect" d20, diverging from the traditional "Ah, crud, I'm dead" d20.