Eero Tuovinen writes very eloquently and with great insight about playing D&D. He is currently discussing his approach and an ongoing IRC game at the Story Games forums here: Writing up Eero's Primordial D&D and here: Old School Renaissance: Actual Play - Greysands Campaign.
Here are some nice quotes (emphasis mine):
I am admittedly beyond hardcore on [the matter of low-level lethality]. I entertain myself thinking up ways to make the D&D support even more meaningless lethality. I find that the constant, nihilistic existential pressure focuses minds wonderfully, and makes the occasional streak of success taste all the more sweet. I simply don't have any interest for facilitating the survival of this particular character any further than his choices, talents and luck take him.
I basically just don't take a D&D GM seriously before they've killed a few PCs. We can talk about it all we like, but until I see them do it, it's all insubstantial theory - maybe their characters have just been skilled and lucky, but also maybe the GM is misusing their wide influence to undermine the supposed dangers. (The D&D GM has such a multitude of influences that it's almost impossible to get a legit game if the GM doesn't want one.) Not that I want my character to die when playing, it's just that I want a legit resolution even more. The only difference between a gauntlet by fire and a puppet theater is in whether there actually is a legitimate possibility of failure.
I should say that while it is possible to make things "too difficult", my experience is that GMs generally vastly underestimate the level to which players are willing to rise. The thing is, if you've already decided that the PCs shall, by and large, live and prosper, then you've already made it impossible for the group to find out how high you can go on the difficulty before the players give up. They'll never have the chance to encounter the setting in all of its true brutality if you as GM shirk away from it. You have to be brave first, or the players never get the chance to be.