Eldritch Warlord said:
Half-life really isn't a cultural phenomenon. Just very popular amongst gamers.
This, Halo is a cultural phenomenon, Half Life is more popular among people who are into games. If you compare Half Life 2 to Halo 2, Doom 3 and Battlefield Vietnam, all released within a couple of months, that might explain it's popularity a bit better.
Anyway, I think the crux of it's success is it's pacing and it's attention to detail. If you start looking at things more closely the length of the game easily doubles.
There are so many more plot details, little toys to play with and spaces to explore than the game actualy signposts for you. It's the complete opposite of most FPS's where straying from the predetermined path yields a big fat nothing, or things that add nothing to the game.
It's also better written and better designed than most games too, if you actually stop and look at them the Combine are fairly original in their designs, although the Headcrabs are pretty generic.