Well clearly you don't know much about the huge variety in FPS games. So is Counterstrike run and gun?
Anyways reasons for Halo's greatness:
-Amazing plot.
-Very, very intelligent enemies.
-Great art direction - just look at the different factions of the Covenant and the architecture.
-Split screen co-op play?!?
-Massive environments.
-Awesome, dramatic levels.
-Teammates who you cared about.
-Great voice acting.
-4 player split screen at a steady frame rate.
-The vehicles controlled brilliantly.
-Tons of great innovations like the gun limit, shields system, the invisible elites with swords, etc.
I could go on for hours. You don't like FPS so you don't get it. That's cool. But you wouldn't find me trying to say all sports games were alike.