This game made the Xbox more popular...other than it's predecessor.
Story: 7.5 (I liked the story over this game's predecessor)
Graphics: 8 (Vastly improved over the original)
Music: 8.5 (Still the same as before, but nothing to complain about here)
Controls: 8.5 (Also improved, even to awe at times)
Sound: 8 (Still the same, nothing much changed)
The story continues with Spartan 117 ("Master Chief") and Humanity's war with the Covenant (A whole bunch of aliens following a few "prophets" and waging war for little to no reason) and continuing the struggles for survival, etc...etc...it's been done before, and to avoid "spoiling" those who haven't yet played Halo 2, or the series in general, this is all I'll explain about the story.
In terms of game-play, EVERYTHING is improved to a positive degree. Dual wielding weapons is now a factor in the series, not to mention newer and better weapons throughout the game, and other details I'm not going to explain (to avoid spoilers), but all in all the game-play is great, surpassing it's predecessor.
This game is a game worth playing, yet I strongly recommend playing Halo: Combat Evolved before Halo 2, in order to know most of the story or even why so many people are playing it, one should play the first in the series before skipping into this one.