I preferred Marathon a lot over Doom. Some reviewers have called it "the thinking man's" shoot-em-up and I think this holds some truth. Of couse Marathon was released almost a year after Doom's release, so Bungie had plenty of time to improve the genre in many areas. Not just in looking up and down, the game also featured:
- weapons with multiple modes and reload mechanics
- advanced lightning effects
- "5D" space (one of the multiplayer levels used this)
- physics (e.g. grenade hopping and missile ballistics)
- neutral and allied npc's
- more varied elevator / switches system (e.g. in the "Colony Ship For Sale Cheap" level)
- motion tracker in UI
- nifty level mechanics, like the levels without air (e.g. "G4 Sunbathing")
I still wonder if Jason Jones could have become an equal to Carmack if he kept his focus on coding games ...