That Old Idea of “Game”
To call D&D an innovative game is a bit like calling the Wright Brothers’ vehicle an innovative car. The definition of game has expanded to include D&D, but there is really no reason that a neutral outside observer would categorize it that way. A definition for game would previously have been something like: A contest between two or more players or sides, constrained by rules that are understood by all players, which ends in a ranking — of winners and losers, or perhaps a score or exchange of money. Oh, and it is engaged with for entertainment. Now, along comes D&D, and what do we have? We may have no winners or losers, the rules may be unknown to the players (only the game master needs to know them) and are often somewhat flexible, and there may be no end to the game. There aren’t even clear sides; there are players and the game master, but the players are not exactly on the same side as their fellow players and the game master is neither antagonistic not usually entirely neutral. The only unambiguous constant with that old idea of game is that D&D is engaged with for entertainment.
— Richard Garfield, Hobby Games: The 100 Best
“The definition of game has expanded to include D&D”. Exactly.