I'm amazed with the richness and integration of data they managed to squeeze into this space that. I changed my avatar online by purchasing a Penny-Arcade pack and it changed in my online gamertag and the Xbox forums as well. I play a game and it shows up in the card. I earn an acheivement... and its in my online profile. Of my non-xbox-dev Friends I have the highest score in Geometry wars... and they see it when they are online and in the game itself. 🙂
I know this stuff isn't rocket science, but I continue to be impressed with the "little things" and details that were paid attention to for the Xbox 360.