Doom didn't invent the FPS - iD's own Wolfenstein 3D did that - but it transformed it overnight into the juggernaut genre it is today. The game mechanics were hardly revolutionary, with the fundamentals of shooting, strafing and keycard collecting making a fairly straight transition from Doom's WWII predecessor, but the visuals were totally unheralded in terms of texture detail, 3D architecture and lighting. Doom was the first game I remember playing that actually looked, well, almost real, and only the ghastly pixilation of the bitmap monsters when viewed up close really spoiled the illusion.
Most of all, Doom ran with the idea that the first-person viewpoint was one that could lead to total immersion. There was something in Doom's atmosphere; in its visual design and in its fearsome combination of 3D graphics, pounding music and audio effects, that made it the most compulsive and absorbing game in town. While titles like Alone in the Dark had already proved that games could be scary, Doom genuinely was the stuff that nightmares are made of. Even Origin's mighty Ultima Underworld II seemed primitive by comparison.
Just as importantly, Doom paved the way for online gaming and the DIY games mod community. At first, multiplayer deathmatch games were limited to office IPX networks, rather limiting the audience to those with access to such facilities and the freedom to use them. However, third-party applets soon cleared the way for deathmatch games over the Internet - a practice that iD would take one step further with the release of Doom's successor, Quake.
Added to this, Doom fans discovered that, with fairly simple, homebrew tools, they could create their own levels and share these with other fans. iD actively embraced the idea, and the modding community was born. Without Doom, the modern gaming world would be a very different place indeed.
Mention: The fearsome cacodemons - gaming's most terrifying tomato-based monster.
Don't mention: The threequel. What good is new tech without new gameplay to go with it?