One feature I'm surprised to see missing (unless I just haven't worked out how to do it!) it to be able to send/recommend videos (either on the dashboard or within apps like youtube) to friends via the xbox 'messages' app.
There are often videos on the dashboard or on YouTube I would like to be able to recommend to friends and it would be great if I could just select the video and hit the menu button then select "Share", pick the friends I wanted to send it to and add a message. They could then open the message and select the link to view it directly without having to find it manually.
This could be expanded to let you recommend music, apps and games too.