Whose idea was it to take a game that barely had a plot to begin with and turn it into a movie? Come on... A 320x240 screen worth of text, followed by thirty-some, key-hunting kill-fests and another screen of text does not a movie make. The only reason anybody can take that kind of gameplay for hours at a time is because they have direct control over the protagonist's actions. An hour and a half to three hours in a theater, just watching, with no control over the action would be utterly unbearable.
Oh, that's even if they had decided to remain true to the game. Instead, as Kalfireth mentioned, they couldn't even stay true to the single A10 page worth of plot material. Seriously, let's take a look at the condensed version...
Doom 1...
Marine goes to a base on Mars. Fiendish beings attack Marine. Marine kills ****, while working ever-deeper into the base. Marine goes to hell. Marine doesn't like hell too much, so he starts killing off the inhabitants. Marine returns from his jaunt into the underworld. Hoo-****ing-ray.
Doom 2...
Fiends in hell don't much care for hell. Fiends come to Earth. Marine kills more fiends on Earth. Hell is driven back to hell, so to speak.
Doom 3...
Marine wonders why the hell people still pay for this rehashed ****-in-a-box. Marine probably kills some more fiends, but really, who cares anymore?
Now, who's the screenwriter who couldn't hold true to that? I want to string him up by his own intestines and light him on fire before he dies, and even then, I'd feel like I was being too merciful. If you're going to make a movie-conversion of the game with the most simplistic, shallow plot EVER, then you have no excuse for "taking artistic liberty." It wasn't art as a game, and it's sure as hell not going to be art as a movie, so there's nothing artistic to it, and when you have such a thin plot, you already have enough room to work with that you don't need to be taking any liberties!
RAWR!
What? You don't agree? You think Doom was a deep, involving game worthy of all the world's acolades? Moreover, you predict this movie will rock the socks off of any movie I would dare to call "better?" Feel free to die in a fire, so that the world may have the pleasure of hearing your screams of agony before being rid of your particular brand of stupidity entirely.
Mmmm... Nothing like a good rant.