DonAdam
Explorer
As far as what I would consider to be core D&D sacred cows (this is not to say that if you change one or two you're not playing D&D):
Vancian Magic- for all its shortcomings, it's very good for a game because of the tactical nature of it. Even the sorcerer, being spontaneous, is a very well done adaptation into the Vancian framework. It also makes new spell creation easy. I especially like it with a bunch of spells from other sources (for wizards, at least). Imagine if each spell was unique to a group of casters or a single caster- now its magic again, because its no longer standardized.
Classes which are at least partially archetypal (as opposed to d20 Modern, in which the classes don't specify much at all).
Hit points and armor class- again, despite their failings to wargamers, these are great simplifying abstractions, and are more powerful as such the less "conditions" you have- for instance, if there was no such thing as "prone" an attack on a high level fighter could be described as a trip, putting him at a disadvantage in the fight.
Fighters, rogues (or thieves, the name is unimportant), wizards, and clerics
Elves, dwarves, halflings (I wish I could include gnomes, but they're not as universal, and these infernal 3e halflings that act like kender have made them obsolete- bring back hobbit halflings and let gnomes be the silly ones).
Orcs, goblins, kobolds, skeletons, and, uh... dragons
Longswords do a d8 damage (feel free to include other iconic examples)
Saving throws (the concept, which falls outside the parameters of AC)
Levels with an exponential power curve, especially for spellcasters, but for others as well with the accumulation of better magic items while abilities improve at the same time
Magic missile, charm person, fireball, wish
Gold pieces, silver pieces, copper pieces
Weapons and armor +x, scrolls, wands
Elven chain, dwarven plate
The Greyhawk "named" items, ie Bigby spells, Vecna's hand, etc
The last two aren't mechanics but...
Dungeons
Taverns
Vancian Magic- for all its shortcomings, it's very good for a game because of the tactical nature of it. Even the sorcerer, being spontaneous, is a very well done adaptation into the Vancian framework. It also makes new spell creation easy. I especially like it with a bunch of spells from other sources (for wizards, at least). Imagine if each spell was unique to a group of casters or a single caster- now its magic again, because its no longer standardized.
Classes which are at least partially archetypal (as opposed to d20 Modern, in which the classes don't specify much at all).
Hit points and armor class- again, despite their failings to wargamers, these are great simplifying abstractions, and are more powerful as such the less "conditions" you have- for instance, if there was no such thing as "prone" an attack on a high level fighter could be described as a trip, putting him at a disadvantage in the fight.
Fighters, rogues (or thieves, the name is unimportant), wizards, and clerics
Elves, dwarves, halflings (I wish I could include gnomes, but they're not as universal, and these infernal 3e halflings that act like kender have made them obsolete- bring back hobbit halflings and let gnomes be the silly ones).
Orcs, goblins, kobolds, skeletons, and, uh... dragons
Longswords do a d8 damage (feel free to include other iconic examples)
Saving throws (the concept, which falls outside the parameters of AC)
Levels with an exponential power curve, especially for spellcasters, but for others as well with the accumulation of better magic items while abilities improve at the same time
Magic missile, charm person, fireball, wish
Gold pieces, silver pieces, copper pieces
Weapons and armor +x, scrolls, wands
Elven chain, dwarven plate
The Greyhawk "named" items, ie Bigby spells, Vecna's hand, etc
The last two aren't mechanics but...
Dungeons
Taverns
