This is not an attempt to start an edition war.
Old School, and What Do We Mean By That?
I think old-style, gritty Dungeons & Dragons can be boiled down in to a simple concept: expectation of survivability or mortality. The current version of D&D puts forth a set of abilities and skills that provide starting characters with heretofore unheard-of added abilities in combat and beyond (the entire "skill" system, for example), and options. In Advanced D&D (or Basic, or Original D&D) the assumption is, for example, when checking for a secret door that you have done all you can to find a secret door. Or likewise for a trap. Magic-users in AD&D have it even rougher - a true by-the-book adherent DM might not roll and give the neophyte mage the "long bomb" of 1st level magic-users, sleep! (You did know that as a DM you can dice for the spells your starting magic-users get, correct?)
Starting out with a 1st level party or character in prior versions of D&D is fraught with peril. Fighters with potentially fewer hit points than the magic-user. Thieves who are sketchy at best in terms of finding traps. Magic users who might be able to lay a swath of low-level monsters out - or maybe not. Clerics who can either make the whole party harder to hit and hope that no-one does get hit, or try to heal up one who is hit. Magic-users start with but a single spell. Average clerics have a single one, but if you're lucky you might get two or perhaps three additional chances to have a cure light wounds in the party.
Not so in 3rd edition. Fighters get many feats. Anyone of any stat spread can take special classes such as Paladin and gain the added bonuses for them as well. Maximum hit points are given to start. All of these abilities assume that even with slightly less than standard play a 3rd edition party of 1st level could plow through most 1e by the book encounters without batting an eye. Thus the challenge is removed, or lessened, from the outset.
Of course I've said before that there are "feats" in AD&D, but these special class abilities are rare: using the methods laid out in the Dungeon Masters Guide it isn't really all that easy to just "roll up" a paladin or ranger or monk, much less a dual-classed character who can change roles at will!
Nowhere in 3rd edition are there instant-kill anythings. Poisons cause a loss of CON - but leave the character alive. In a game where stats can be "bought" (and thus elevated into the for-1e-stratospheric high teens), is the loss of as little as a couple of points of CON any real threat? I say no.
Yes, in 3rd edition, the assumption is one of survival. "Well not in my game," you might say, but the rules of this current edition of D&D don't bear that out. A first edition AD&D character who survives by dint of luck, thoughtful gameplay, and tactics has survived. A 3rd edition character has an arsenal of powerful tools (that continue to grow in power) and his survival is merely expected at low level. Even at high level, AD&D characters can be slain. Consider the 10th level fighter who, on the average, has 50 hit points. Squaring off against a very old red dragon with a breath weapon, a missed saving throw (and at 10th level, our fighter is only avoiding half of that damage 55% of the time, which means that the worm might have to employ a 2nd blast) means death to our venerable fighter! And that fighter is doing the same amount of damage to that red dragon in return that he was doing at 6th level, 8th level or 9th level.
The expectation that the world at large is going to kill your character is the key difference: that death certain waits in the shadow of ancient forests, unexplored ruins, haunted castles and abandoned cities. There are not minor skills to ablate, there is no "take 20" - a deadfall is located, or it is not.
That is the answer, my friends. Mortality.
