Alien enemies, drivable vehicles, long open-area levels, and carry limits were already being used in other first person shooters. Halo, to me, was like Rainbow Six, Goldeneye, and Half Life all rolled up into one. It didn't seem as original as you are making it out to be. Combining the best of these was very appealing, especially with the new physics engine available on a console. Rainbow Six - Carry limits with tactical prep and execution Goldeneye - Drivable vehicle Half Life - combined shield and health system (Even Doom had this)