I think it would be very helpful if Google Reader could have a "suggestions" item which would present you with stuff to read from places that you might not even be subscribed to.
It could do that checking who else is reading, sharing and starring similar stuff with you and putting these people as your neighbours. Then, your suggestions would be stuff that your neighbours shared and that you're not subscribed to. This would allow people to discover items that they wouldn't most probably see, even from topics that they might not normally have an interest in
The idea is similar to the way last.fm does it in order to suggest new songs and show you what your neighbours are. Features that I've found very very useful in order to discover new music I'd like.