When it was released on the Xbox in 2001, it combined many elements not usually seen in first person shooters, especially on consoles. There weren't many shooters back then that had even just one of the unusual FPS tropes. The ability to melee with your weapon, being limited to holding just two, regenerating health, seamless vehicle combat, grenades being a separate ability, etc... I could go on. The most ardent and embattled PC gamer in response will naturally make a laundry list of PC games at the time that may have just one or two of said features but they won't find a game that combined all of those and did it so well at the same time.
Then they'll go on to say 'Halo sucks because I SAID SO'.
It's also highly erroneous to bring up a console game when we're debating shooters for the PC. Personally it's refreshing to see Modern Warfare 2 or DRM, or dedicated server functionality being the new bogeyman of PC gaming
