MoogleEmpMog said:
To me, D&D is classes (probably this has to include the roles of the fighter, rogue/thief, wizard/magic-user and cleric, though the names can and have changed), levels, mind flayers and beholders. Dungeons and dragons have to exist somewhere in the world - hence the name.
And above all, the kitchen sink has to be thrown in, the serial-numbers-filed-off amalgam of the best, and often the worst, that fantasy has produced to date; jotuns and trolls living down the lane from titans and centaurs, with kirin and hopping vampires their neighbors in the next celestial county; not Conan OR Gandalf OR Hercules OR Cloud Strife, but the four of them teaming up to kick ass and chew bubblegum, possibly with Batman's help. Crashed spaceships give our heroes rayguns with which to shoot balrogs (er, balORS), dinosaurs live on lost plateaus overlooking quaint medievalesque villages, worg-riding goblins fight Stygians, and the Argonauts compete with the Knights of the Round Table for the Holy Grail - until the PCs show up and steal it away from both because they need it to kill Cthulhu.
Any given campaign may not incorporate even a fraction of these elements (and most are almost certainly better off not doing so

), but the game needs room for them.
That's the CORE of D&D to me. Classes, levels and the kitchen sink.
I rather like this phrasing of things.
To put it simply, D&D is D&D so long as you can express 50-90% of your character's capabilities, potential, and flavor with the expression: my character is a [insert ordinal number] level [insert race] [insert class]. Having a fifth level human paladin is the very core of D&D. Rolling a d20 to achieve either total failure or total success, with a small chance of critical success (as opposed to Alternity-style degrees of success), is also essential. If you pay attention, whenever D&D shows up in a pop cuture reference, it is
always going to involve one of those two elements.
As a more rigorous definition:
D&D is a tabletop game based around a group of players, one of whom is desginated the DM, who controls the game, with the rest acting as players who each control one character (usually). There is no pre-set board to play on, and no standards for miniatures or playing pieces. All actions are resolved with the throw of dice, most commonly the d20. d20 rolls are absolute binary rolls for success or failure, and most other rolls are done to assign a "damage" number. Characters are constructed by choosing ability scores, a race, and assigning levels to classes. Levels are gained through the gathering of XP, primarily from defeating monsters controlled by the DM and overcoming other challenges. The game is most often set is a generic fantasy setting (with several options to choose from), though variants exist to place it in other settings (like d20 Modern, which I would say is a mere variant on D&D, not something seperate). Many common assumptions are that there are a pantheon of gods, angels, demons, dragons, various intelligent species, magic which can be controlled by learned individuals, magical items, and a wide variety of creatures and concepts taken from myth, fiction, and random inspiration (or the lack thereof). There is a wide range of IP associated with the game, such as creatures called Mind Flayers and Beholders, though this gets robbed by Japanese videogame companies on a daily basis. There are certain cosmological assumptions of "other planes of existence", though interpetations vary. All is subject to DM reinterpretation, and house rules.
Whew... I hope that covers it.
It is a minor point, but there are a number of minor flavor and mechaical assumptions that are pretty distinctly D&D, but they are far more fundamental than what most people are complaining about. Stuff like Clerics being able to cast spells, the arbitrary distinction between demons and devils as different kinds of being (why not just call the mooks demons and the rulers devils? it is an equally valid and arbitrary choice, but 4E not doing that, it is sticking with the clsssic assumption), different size categories, restricting weapon and armor choices based on class, restricting magic use based on class (rather than giving magic to specific races, or giving it to everyone), etc. There are a
lot of fundamental flavor and mechanical changes they could make that would make it "not D&D", but nothing announced so far even hints that they will do so. As a whole, the changes people are griping about are things that are not fundamental assumptions, they are just specific details and implementations.
Well, I admit the death of Vancian magic is somewhat major, but 3E had been eroding away at that one for years, so it lost its importance long ago.