"From The Sorcerer's Scroll: D&D, AD&D, and Gaming," by Gary Gygax (The Dragon #26, June 1979):
Because D&D allowed such freedom, because the work itself said so, because the initial batch of DMs were so imaginative and creative, because the rules wre incomplete, vague and often ambiguous, D&D has turned into a non-game. That is, there is so much variation between the way the game is played from region to region, state to state, area to area, and even from group to group within a metropolitan district, there is no continuity and little agreement as to just what the game is and how best to play it. Without destroying the imagination and individual creativity which go into a campaign, AD&D rectifies the shortcomings of D&D. There are few grey areas in AD&D, and there will be no question in the mind of participants as to what the game is and is all about. There is form and structure to AD&D, and any variation of these integral portions of the game will obviously make it something else. The work addresses itself to a broad audience of hundreds of thousands of people—wargamers, game hobbyists, science fiction and fantasy fans, those who have never read fantasy fiction or played strategy games, young and old, male and female.
AD&D will eventually consist of DUNGEON MASTERS GUIDE, PLAYERS HANDBOOK, GODS, DEMI-GODS & HEROES, and MONSTER MANUAL and undoubtedly one or two additional volumes of creatures with which to fill fantasy worlds. These books, together with a broad range of modules and various playing aids, will provide enthusiasts with everything they need to create and maintain an enjoyable, exciting, fresh, and ever-challenging campaign. Readers are encouraged to differentiate their campaigns, calling them AD&D if they are so. While D&D campaigns can be those which feature comic book spells, 43rd level balrogs as player characters, and include a plethora of trash from various and sundry sources, AD&D cannot be so composed. Either a DM runs an AD&D campaign, or else it is something else. This is clearly stated within the work, and it is a mandate which will be unchanging, even if AD&D undergoes change at some future date. While DMs are free to allow many unique features to become a part of their campaign—special magic items, new monsters, different spells, unusual settings—and while they can have free rein in devising the features and facts pertaining to the various planes which surround the Prime Material, it is understood they must adhere to the form of AD&D. Otherwise what they referee is a variant adventure game. DMs still create an entire milieu, populate it and give it history and meaning. Players still develop personae and adventure in realms of the strange and fantastic, performing deeds of derring-do, but this all follows a master plan.
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I was browsing some PDFs from the Dragon Archives and came across this From the Sorcerer's Scroll column by Gary. Therein, Gary perfectly describes the situation with D&D at the time AD&D was published. I just thought I'd point out how his "non-game" description perfecty describes the state of "2nd Edition" by the mid-ninties, with the proliferation of "splatbooks" and other rules expansions.
I can attest that no two groups played AD&D the same. Most DMs and players, rather than playing AD&D, "played at the game" (as Gary once said in Poker, Chess, and the AD&D System), making a game out of tweaking and cherry-picking proficiencies, kits, and so on from the various books to make a mish-mash of a "system." As a DM during that era, I constantly had to fight players that wanted to incorporate this or that from other sources (things like the Bladesinger) that really disrupted the core of the system and originated from sources that certainly were not playtested.
"While D&D campaigns can be those which feature comic book spells, 43rd level balrogs as player characters . . .": It seems we have come full circle . . . the original D&D game allowed for super-characters, and now the d20 version does much the same.
This truly shows the insight Gary had into the nature of the game!