Probably the key turning point toward modern gaming. I can still hear the screams now, games made you anxious before, but this one could make you scared!
The game was revolutionary for many reasons. For one it was released for free (as shareware), with the full game later going on sale, so everyone got a taste of the fun. Though not so many sold, it became a legendary franchise (which wasn’t free anymore $$$).
In the beginning of 1993 Id software bragged that Doom would severely decrease productivity around the world. Which went on to be true, not only of employees time but also decimating computer networks with deathmatches.