The Reader secret sauce for me was the friend-of-friend social model. You shared items with friends, they shared items with you, and if your or their friends' friends commented on something shared, you/they could see them too. It made it easy to organically grow a close and active group from people who already had a social connection to you — "hey, that person posts lots of interesting comments and knows someone that I follow, I'll follow them too". (And if you didn't like someone or their friends, you could unfollow or block them.)

Maybe it's kind of there in how instances interact, or the post visibility features, or some of the forks like Hometown, but Reader had that right-feeling combo of simplicity, personalization, and built-in network.