I'm not one who is into the whole "video games" craze, (and it's not allowed in my house either) but I remember when Halo "1" first came out, and the obsessiveness about it... Halo 2 now shows up and has taken over the popularity charts with it's not-so-new alien fighting, and player-against-player fighting. And, indeed, the graphics have since improved.
Now, I may not really be "all knowing" about such games as this (cause I find it not really exciting) but isn't it really the same thing with any other battle-one-another and fight-against-aliens game? what makes Halo 2 such a grand hit?
Probably an improved engine. Every year the coders try and take the level of gameplay up a notch, Half Life2 is not that great from a mission standpoint but the level of realism placed in the characters and surroundings make up for it.