I believe we need to separate the D&D brand from the classic or traditional D&D system, and it sounds like [MENTION=6799753]lowkey13[/MENTION] is focusing on system. He touched upon the major cornerstones (hit points, saving throws, classes, ability scores). To that list I would also add some form of a single die roll/opposed roll core mechanic which would encompass armor class and skill resolution.
Aside from the classic mechanics, I believe "traditional" D&D empowers the referee with nearly all of the narrative control outside of direct player agency through the actions of their respective PC. This is a notable split from games like Dungeon World which grant players much more narrative control of the world around them. This could certainly change in the future. Even 5e has some player empowering variants, but they're only given very minor lip service as side options in a book that's not meant for players.
To that end, I consider Pathfinder and a lot of the OSR variants to be D&D in spirit. The window dressing might be different and some mechanics might be tweaked, but the GM/Player relationship and conversation is largely the same and anyone familiar with classic D&D should not have a hard time fitting in at a new table.
Aside from the classic mechanics, I believe "traditional" D&D empowers the referee with nearly all of the narrative control outside of direct player agency through the actions of their respective PC. This is a notable split from games like Dungeon World which grant players much more narrative control of the world around them. This could certainly change in the future. Even 5e has some player empowering variants, but they're only given very minor lip service as side options in a book that's not meant for players.
To that end, I consider Pathfinder and a lot of the OSR variants to be D&D in spirit. The window dressing might be different and some mechanics might be tweaked, but the GM/Player relationship and conversation is largely the same and anyone familiar with classic D&D should not have a hard time fitting in at a new table.