You may be the person who cringes when they hear of sequels. It is generally expected that the sequel will not be as good as the original... it is not the case in this instance.
Halo 2 is everything Halo was and more. Excellent campaign? Check. Excellent multiplayer? Check. New weapons? Check. Better graphics? Check.
I may be in the minority on this, but I thought Halo 2's story was AMAZING. The plot twists and turns and leaves you hanging until the end with HUGE surprises that I won't dare spoil for you.
The multiplayer is much as it was in the original, except you can now play on XBOX Live, a GREAT addition. The party system doesn't work quite as well as advertised but it doesn't stop the online multiplayer from being extremely fun!
The graphics are MUCH improved and the landscapes you fight in are very well done.
The biggest addition to the game was Duel-Wielding. DW adds a completely new dimension to the game. Close quarters combat now takes a little more thought and strategery rather than "Well, I'll shoot him with the Assault Rifle or Pistol!" Now you'll need to experiment with some different combinations to see which one fits you best!
I truly believe that this game is better than its predecessor (not by much mind you) and that's saying something. Bungie has done a great job creating a feasible sci-fi universe. You'll keep playing the single player campaign (which is longer) just to see what happens next.