With all the excitement surrounding Halo 2's release, it seems that the new features added to Bungie.net's website have been overlooked. A co-worker told me about it, and I am now in awe.
Once you finish a game over XBox live, all your stats are uploaded to the servers. Sure, you get all the standard stuff like number of kills, deaths, etc, but there is much, much more to be explored here.
Halo 2 introduces an experience system so that you get to play against players of a similar skill level. You can view your current experience level in each of the different game types on the Bungie website, as well as your ranking compared to your friends.
There are also "medals" which are awarded for doing specific actions in the game, like carrying a flag, planting a bomb, killing the flag carrier, etc. It's nice finally getting credit for something other than kills!
In addition, you really should check out the new "Game Viewer". The Game Viewer shows you graphically who you killed, from where, to where, and vice versa. If you can't figure out where you keep getting sniped from, this is the tool to expose the assasin.
Here's an article that has screenshots, or you could just go ahead and create an account on the site to experience it for yourself!