The biggest innovation of "Doom" was something subtler. Video games, then and now, are mainly passive entertainment products, a bit like a more interactive television. You buy one and play it until you either beat it or get bored. But "Doom" was popular enough that eager users delved into its inner workings, hacking together programs that would let people build their own levels. Drawing something in what was, essentially, a rudimentary CAD program, and then running around inside your own creation, was an astonishing, liberating experience.
Being able to alter the game transformed the player from a mere passive consumer of media into a producer in his own right, something that is much harder in most other kinds of media. Amateur filmmakers need expensive kit and a willing cast to indulge their passion. Mastering a musical instrument takes years of practice; starting a band requires like-minded friends. Writing a novel looks easy, until you try it. But creating your own "Doom" mod was easy enough that anyone could learn it in a day or two. With a bit of practice, it was possible to churn out professional-quality stuff. "User-generated content" was a big buzzword a few years back, but once again, "Doom" got there first.
