DOOM was released. And the world would never be the same again.
DOOM was remarkable as a computer game in four significant ways:
1) It was the first widely-popular First Person Shooting game (FPS).
Yes, there were others before it (Catacomb, Wolfenstein 3D), but it was DOOM that popularized the genre and changed gaming. The sense of being in the game was unlike any other on the market. The only other widely player first person types of games at the time were car racing games. DOOM added fear and paranoia, sound and lighting to make gameplay seem almost real.
2) It proved that shareware was a viable and profitable method of distribution
DOOM's shareware release (today, it would be called demoware) meant people could download the program and play before paying, able to get a taste without buying the whole meal. And because the meal was so tasty and new, people did pay. Sure, there was a shareware market and distribution system, but it was used mostly by individual programmers, not large computer companies who continued to sell programs in packaged boxes.
3) DOOM introduced modding to the world
Other people had modded games before id (e.g. Ms. Pac-Man), but DOOM made it easy for less technically proficient people to create and insert their own graphics, sounds and level design into the game, something never before possible for most. Modding spawned (pun definitely intended) its own industry of add-ons that people made and sold.
4) DOOM introduced networked gaming
Before DOOM, "multiplayer gaming" meant either taking turns (playing until you "died" and then handed the controller to another player until that person "died") or two players looking at the same screen with the same view (or worse, a top/bottom split screen with a smaller view).
DOOM's appearance meant that two people could now play against each other with full computer power to themselves. And not just separate machines, but separate viewing and playing. It meant you could play someone across a room, in another room, another building and eventually, another country. DOOM make the internet a near necessity for multiplayer gaming.
Whether you like DOOM or not, you have to respect its place in history and its influence. Just as the Lord Of The Rings movies could not exist without Dungeons and Dragons, so too could today's gaming not exist without DOOM.
DOOM is likely to remain popular for the next decade given John Carmack's decision to release the source code. There are many high quality ports of DOOM still in development, with additions of sound, graphics, player movement and many other features. And with the plethora of fan-made WAD files to use, there's no end to customization. DOOM is nowhere near the end of its playability.
DOOM World has a list of ports to various computer systems including DOS, Windows, Macintosh, Linux and several others. ZDOOM, Brutal DOOM, and Chocolate DOOM are among the more popular and recently updated versions.