Oddly enough, Pathways into Darkness was also made by Bungie, and was the first FPS they ever made. It even has a mention of the Jjaro, who played a vital role in the Marathon trilogy and are widely assumed to be the 'Forerunners' that made the Halos.
Doom was more popular, Marathon was a better game. That's how it normally works.
Personally, I found the shotgun in Doom to be less satisfying than *dual-wielding* double-barrelled shotguns in Marathon 2. With those babies, you could cut a path through any alien horde.
And no FPS has yet to have the same level of humour generated by the assimilated BoBs - genetically engineered aliens who looked identical to the ship-board civilians and would run up to you and explode. Remember the map "God will Sort the Dead" from Marathon 2? How many people gave up on trying to tell the real BoBs from the Simulcrons and just ended up blowing everyone up?http://www.davidcsimon.com