This is a thread I fear making, because I don't know how many nerds there are. Dungeons & Dragons was very important. It is the game that created the roleplaying game genre. It's mechanics are the basis of video games, such as XP, leveling up, races, classes and the idea of adventuring as a character. Indeed, it was what inspired some of the earliest video games and lead to major franchises. Games such as Ultima would not exist without it, and without D&D and what it inspired, you wouldn't have things like Warhammer and Warcraft. I'd also argue without those early computer games, you would not have seen the GUI interface on computers. It was Dungeons & Dragons and Star Trek that were the driving engine for nerds, who went on to be nerds that were successful and influential.
And from a cultural perspective, D&D inspired a lot of popularity with the fantasy genre. And it offered a boogeyman for the 1980s. A boogeyman that may have helped Tom Hanks' career (Mazes & Monsters).
So what if it had never existed?