Other than being a generally well-designed feed reader, my favorite feature is that you can configure it - on a feed by feed basis - to render the post website itself (which it trims down to the specific post), rather than the snippet - so with (at most) one click per subscribed feed, I read all the articles inside my feed reader, whether the RSS feed contains an entire article or just a snippet + "click here for more".
Killer feature for me is the ability to group your feeds then making them availale via separate rss channels.
The mobile client is also pretty damn good.
It's also open source for those that want to try to directly contribute code fixes/ideas, but I've rarely felt that need. I'm happy enough just paying the Premium subscription every year and knowing that suggestions on the forums (used to be UserVoice, now is Discourse) get responses pretty quick.
Also, link deduplication, because sometimes articles from the e.g. the CNN Politics RSS overlap with the CNN US RSS