I think joystiq says it best in their article:
Halo 2. Reviews. Need we say more?
The most interesting revelation for me has been the inclusion of bungie.net‘s huge new online stats system. Rumour has it that after you finish an online game, you’ll be able to log on to bungie.net (using your PC), and review the stats for the game, plus (here’s the good bit), view an overhead diagram of the map, showing the path of the flag, or the location of all your kills, etc..
In essence, you’ll be able to get an in-depth view of exactly how you got your arse handed to you by a bunch of 13 year old twitch gamers. Excellent!