We've all seen people we know listen to cool stuff or very silly stuff. It would be awesome to "like" what they are listening to. That is, on the right side of the screen, where I see my friends' activity.
Even cooler would it be to leave a short, temporal comment simply poking at them when they listen to something funny, embarrasing, silly or plain bad. It would make the experience more interactive and engaging.
For example, take a guy who likes metal or rock and all of a sudden you see him listening to The Backstreetboys or Justin Bieber. You don't know why that is. Maybe his sister or his girlfriend are using his account, or maybe they are at a party. One cannot simply overlook. I would love to make fun of people because of this. I've contacted them afterwards via facebook, but it'd be awesome to be able to do it on the spot. It would also be cool if other friends in common could see this and add in to the poking.
People can always choose to listen to music privately, so it's not like people's lives would be destroyed because of a little fun at their expense.