Doom was arguably the killer app of the MS-DOS world. Whole essays have been written about the game's freakish popularity and immense appeal, let alone the game itself. Just about everything good and bad about Doom has been written and rehashed many times, so one more review can hardly hurt.
It's nothing short of amazing how transcendental the game has become. It has a plot you could fit on a postcard, and gameplay that owes much to 2D arcade shooters. Yet many people take it as seriously as the air they breath. It features a faceless, nameless main character who never speaks a word (he's been nicknamed "the Doomguy" by a whole generation of gamers) who has somehow become a more recognizable character than the leads of most RPG and adventure games. It's stupid, brainless, violent and symbolizes every bad impression the general public has about video games. Yet very few games can claim to have transformed a genre, and for that sort of appeal the game must have done something right.
So in my opinion what are the factors that made Doom popular?1. Gameplay.
Doom is the distillation of the FPS genre. No subtleties, no niceties, nothing so pretentious as a story, just crazy white-knuckle action from beginning to end. This is the same formula that 90% of all action games have tried to succeed on. Id Software crafted a game that is extremely fast and extremely addictive, and combined with an advanced 3D engine was almost a sure-fire megahit. Play most 3D games made before Doom and you'll throw them out the window in no time, but Doom is eminently playable even to this day.
But Doom is not simply fun because of arcadish button-mashing, it was one of the first action games you could actually get immersed in. It's difficult to define, Doom just hits all the right psychological buttons. You feel terror, you feel elation, you feel exactly what the game wants you to feel. The game has no story, but that doesn't matter. The graphics (these days) are crappy, but that doesn't matter. While playing it, you are a space marine, trying to escape from a sick and deadly industrial world. It's amazing how much this game involves you in what's happening, especially next to games with much better graphics and much better stories that nevertheless fail to engage you in quite the same way.2. Technology.
Doom wasn't THE most advance game released up to that point (Ultima Underworld had a significantly better engine, with slopes and stackable rooms) but it still was quite revolutionary for its time. Doom introduced gaming to things like spiral staircases, elevators, crushing ceilings, and the most advanced 3D architecture ever seen in a game. It was fully texture mapped, featured realistic lighting, and still ran very fast on a 386. You could even interact with your environment to a limited extent (raising/lowering elevators and stuff like that in realtime 3D, which sounds incredibly simple today but was almost unheard of then).
The game was also designed with a very open-ended structure, allowing (and encouraging) users to add their own content. They even released the source code. Even if you don't feel like downloading one of the numerous Windows ports of the game, the game generally works fine on Windows XP. Doom is a technological marvel.3. Ambiance.
Doom is not a horror game, but nonetheless contains many scary moments. It was the first FPS to realistically incorporate lighting. If you fire a gun, everything around you gets lit up by the muzzle flash. Lights could fade in and out like strobes, and some really tense moments occur when there's hardly any light and you can't see what's attacking you. Speaking of getting attacked, Doom's monsters are deformed, drooling, nightmare-inducing freaks, and even after years of playing you can still get tricked into a "WOAH CRAP!" reaction when one sneaks up on you. The game's textures are weird and in many cases disturbing, some of them are miniature works of art. Lastly is Bobby Prince's unconventional but very effective music, which complements the game's frantic, gut-wrenching action and tense, claustrophobic levels extremely well.4. The right place at the right time.
OK, it's fun to have the illusion that Doom was The Little Engine that Could, a tiny shareware game that succeeded against all expectations, but that's not the way it happened. The stars were aligned for Doom's release. Firstly, technology had advanced to the point where texture-mapped 3D games were possible and playable. Second, modems and netcards were starting to become common, giving Doom a very powerful selling point in its multiplayer. Third, the groundwork for how the FPS genre should play like had been laid down by Wolfenstein 3D. The gaming industry was waiting for something that exploited those concepts. Doom's creators knew exactly what they were doing when they made it.
It bears mention that Doom was the GTA3 of the time in terms of controversy, due to its extreme violence and satanic references. By today's standards Doom looks tame, but it was definitely a factor in the rise of what we now call "adult" games.
You could argue that the difficulty is skewed. The first four levels are on the easy side, and the fifth is a joke (I mean that literally, the guys at id confirmed it at one point). Also the game can sometimes be a bit too dependant upon mazes and key-hunting instead of action, though far less so than Wolfenstein 3D. This is all just nitpicking, and at the risk of sounding fanboyish there really isn't much wrong with Doom.
With that said, you need to approach it with the right expectations. Doom is incredibly advanced for its time, but the fact remains that it was made in 1993. If you play current-gen shooters and have gotten used to scripted cutscenes and inventories and auto-scaling difficulty and bullet-time effects then you'll have to lower your expectations when you play Doom, otherwise you're setting yourself up for disappointment. Fair warning.
The Bottom Line
Doom is a great game and I recommend it without hesitation. It's the most influential shooter ever made and even today it's romp to remember (provided you make some concessions to it). I'll leave you with this interesting note. The Doom community these days is not made up of nostalgic oldies or die-hard collectors like you'd think, but rather gamers who choose to play Doom over new releases. The game exercises a strange charm that transcends age in the same way Pacman and Tetris do. Think about all the hyped new releases as of this writing: Gothic 3, Quake 4, Call of Duty 2, etc. Will anyone be playing them in 10 years time? Maybe not, but people will still be playing Doom, I'm almost sure of that.