They're FINALLY allowing people to do voice chat outside of a game with more than 2 people at a time! Up to EIGHT in fact!
Another big feature that's awesome for me, is that now people with Netflix subscriptions can watch streaming video rentals (which normally you do on your PC using IE) on your TV via the 360!
What's more, these two features I'm hyped about can apparently COMBINE. You can watch a streaming video with up to 7 of your friends while in a voice chat! The only question mark I have on this, is whether or not all people in the party have to have a Netflix subscription are not. If they do, it's a cool feature. If they don't, it's an amazing feature.
This article only hints at the Netflix part, but I was reading another earlier that went into further detail.