It was Quake-level game tech but released just before Doom 2 came out, if I recall. You could look up and down, grenade jump, use dual-fire weapons, all sorts of very funky stuff that was years ahead of other FPSs. By the standards of the time the controls were massively complex (meaning you needed more than the seven bindings that were necessary to play Doom like a pro). Two triggers
Haven't played it in a decade - might need to give it a whirl when I'm done with STALKER.
The tech was definitely ahead of its time (Halo still contains an evolved version of the Marathon engine), but Marathon was loved because of its ambience, story, and incredible multiplayer. The game made you feel like you were there, and it boasted an intricate plot that I have yet to see any shooter match in terms of complexity.