Just Plain Fun, without the modders and delevelers
I really like some of the sound that comes from the game. What I don't like is having to hear the same song over and over again. I understand that in a game that is constantly giving you directions, it is hard to give custom soundtracks, but come on, they can at least add some music to fit the mood now and again.
The ultimate Xbox Live experience. If you own Halo 2 then Live is a given. In split screen you lost the ability to play against bots, but why play against bots when you can play with people from around the world. They also added a ranking system that tells you what level you are based on your experience points.Sometimes levels can get annoying cause you can finally get the level you want, and then you'll go up against some MODers and lose that level. If you don't want to worry about the whole level thing then you can play custom games with the friends you have made in matchmaking. Customs games have been revolutionized and now you can change every detail of the map. You can even add Turrets!.
Overall this is probably the #1 game for Xbox. But don't take my word for it. Go rent it and try it out. If you like then buy.