While it's story and singleplayer are not on par with the Orginal, Halo 2's multiplayer make up for its short comings.
Halo 2's story focus more on the Covanant. You even get to play as an elite.
New to Halo2 is dual weilding, and vehicle boarding, which add to the fun of the game. The airtight from Halo is back in Halo2.
Some new weapons and vehicles are here in Halo2, but they are nothing to crazy about.
Halo 2 's graphics improve on Halo's already great graphics, however there is some uglyness to it. Halo2's graphics hae some pop-in when it comes to texture mainly due to the limitations of the aging Xbox hardware.
Halo's trademark music also makes a return. It is still some of te best music around.
The big addition to Halo2 is its amazing online play over XBL. Bungie has outdone themselves with the amount of options and game types in Halo2's online play. DLC adds more life to the XBL play. The only down side to the online, is the amount of cheating that takes place. Bungie is doing their best to stop all the cheating. The updates to the software have helped ease the cheating.
Overall, if you have an Xbox, you should buy this game. If you have XBL, you NEED this game. You will spend most of the games life online.