What separated D&D from other games, thereby spawning a new category—role-playing games?
Now, this can be a bit tricky because whatever elements you come up with, you can likely find pre-D&D examples of. But here it goes anyway.
- Non-zero-sum. One player “winning” doesn’t mean another player “loses”. Players can—and typically do—coöperate.
- A referee who—instead of moderating between the players—provides flexibility that no set of written rules can. The scope of the game becomes limited only by imagination.
- Open ended. There are no victory conditions.
- One player plays one “figure” that represents one character.
One was definitely something (according to Heroic Worlds) Wesely was going for with the Braunsteins.
Two seemed to be a tool he discovered—from Strategos—to help make it happen. (It has been said that Strategos got it from Free Kreigspiel.)
Three seems to have developed in Arneson’s Blackmoor campaign.
And again, these things are kind of tricky. I’m sure things weren’t nearly that clear-cut.
Four seems to have occurred in the Braunsteins as well. Although it seems essential to list it, I’m tempted not to. I’m not sure why.