For the vast majority of people out there, whether they be non-gamers or less dedicated gamers than those on this board, D&D = Role Playing Games and Role Playing Games = D&D. You know the type... "What are you doing?" "We're playing [insert non-D&D RPG here]" "What's that?" "It's like D&D." "Oh, O.K."
We could probably define it a little less broadly and confine it to fantasy RPG's, and most people would be pretty well-served with that definition. Really, what is it about D&D that differentiates it from other Fantasy RPG's that wouldn't cause a non-connoisseur to go all glassy-eyed and wonder what the heck you were talking about? Even some of the distinguishing characteristics - "herioic fantasy", "players banding together to acheive a common goal", etc. - cited by people on this board, who I'd imagine know a lot more about rpg's than a general sampling of the public, aren't stating anything that really makes D&D any different from any other fantasy rpg out there.
So back to the question, is there a point in which a company could change the game, slap the "D&D" logo on the front of the book, but have the game rejected as not D&D, but rather some other fantasy RPG by those of us who choose to distinguish? Certainly. And to some, myself and Diaglo to name two, the current incarnation has already gone beyond that point. Of course, others may decide to take a different ideal as their concept of what D&D is, whether that be AD&D1, 3.5e, Hackmaster, or the house-ruled monstrosity they've been tinkering with for the last 25 years.
However, I choose the original boxed set as my basis of what D&D is, simply because if that's not D&D, then what is? So, going back to that original work, what are the prime characteristics that distinguishes it from other games?
1. Some simple generalities that are assumed, but probably shouldn't be - one referee and many players; a semi-cooperative excercise on a part of the players to overcome the obstacles put in front of them by an impartial referee; no set board or playing area, but rather an ever changing field of play as determined by the referee
2. Hard class archetypes, which determine the skills and abilities of the pc's.
3. PC's are rewarded for success with XP. When enough XP is earned the character earns another level in their class, increasing the powers and abilities of the character.
4. Abstract combat in which many diffferent factors go into calculating such things as hit points, armor class, and saving throws.
5. The Vancian magic system, emphasizing forethought and planning, rather than random blasting. Spell casters began weak and grew exponentially in power. They were encouraged to creatively research new spells and create magic-items in order to get around their limitations.
6. PC's should be interested in gaining temporal power - acquiring followers, hiring mercenaries, building armies, owning land. Almost all rules regarding PC-NPC interactions are framed in this light.
7. Not necessarily rules-lite (there were about 110 pages in those original books), but rules flexable. It was designed under the assumption that the referee would do heavy house-ruling and ad hoc-ing. (Any doubters as to this statement should read the Forward on page 4 of Men & Magic.) Rules gaps aren't mistakes, but rather places for the referee and players to get creative.
8. A predominantly gamist bent. The players' goal was to gain wealth and increase their characters' power, i.e. win.
Things that I don't think are integral - the six abilities were almost irrelevant to the original rules, alignment was pretty much an afterthought, setting. The Thief, Paladin, Good/Evil, and Magic Missile weren't even in the game yet, and thus can't be defining characteristics of the game.
R.A.