Features, performance and, later, social network. I used standalone readers but they were poorly suited for casual use – fire it up, wait minutes for every feed to be checked, etc. – and they require a lot of local storage + CPU to do things like store history, determine whether an item has substantially changed or had only cosmetic corrections, etc.
Reader used Google's infrastructure so it was always running and your feeds were up to date when you opened them (unlike most desktop readers, they also adjusted the schedule automatically so frequently updated feeds were polled more frequently), and they stored everything with first-class search for history, which also came in handy if a popular site was overloaded since the cached feed content could still be viewed.
Finally, the real jump in usage happened when they enabled social features. Being able to see who else had publicly shared or commented on an item was a great way to find people with similar interests and there were many great conversations to be found. See http://www.buzzfeed.com/robf4/googles-lost-social-network for a rundown of just how popular Reader's social features were. I believe a key reason why Google+ never went anywhere was because Google actively killed the Reader network and a large percentage of early adopters, journalists, writers, etc. got the message that you should never invest more than transient time in a Google product.
"frequently updated feeds were polled more frequently"
Even better, they supported pubsubhubbub, and were a very early adopter. I think even now, most readers don't support this - it's one of the reasons I settled on Newsblur as a replacement. When I tested with my own feeds, I saw new items appear within seconds of publishing them.
I doubt it'd come back from the dead but if they put some effort into making sharing / finding friend's shares in, you might see something similar to separate things to read from status updates and lunch photos. After, of course, unbreaking the social model which won't happen quickly.
Reader used Google's infrastructure so it was always running and your feeds were up to date when you opened them (unlike most desktop readers, they also adjusted the schedule automatically so frequently updated feeds were polled more frequently), and they stored everything with first-class search for history, which also came in handy if a popular site was overloaded since the cached feed content could still be viewed.
Finally, the real jump in usage happened when they enabled social features. Being able to see who else had publicly shared or commented on an item was a great way to find people with similar interests and there were many great conversations to be found. See http://www.buzzfeed.com/robf4/googles-lost-social-network for a rundown of just how popular Reader's social features were. I believe a key reason why Google+ never went anywhere was because Google actively killed the Reader network and a large percentage of early adopters, journalists, writers, etc. got the message that you should never invest more than transient time in a Google product.