I know Reddit and StumbleUpon and all those cats are good for content discovery, but when you could share amongst friends within Google Reader I discovered so much more, primarily because I have a wide range of friends with truly divergent tastes and good eyes for spotting great stuff. It was like magnifying the number of feeds you had by the Nth degree without over stuffing your own feed.
Now yes, you can share to Google+. And I will concede that it's stream interface is really pretty. But not nearly as many people use it to share out what's in their feeds. Heck. Most of my friends share zilch on there. It's simply not as easy or intuitive to use as the old Reader sharing with friends mechanism.
So yeah, I miss that.