Some people just liked the UI and features, but I think major reason why people are so butthurt about Reader, is that its existence was a very important symbol: it was a mainstream product from a well-respected company that elevated RSS to being a normal feature of websites, not just a niche protocol of open-web-loving bloggers.

And conversely, shutdown of the Reader was a wakeup call that big corporations exist to make money, and things that don't monetize well are not going to survive.

That was just past the peak of "Web 2.0" frenzy that used free public APIs for all kinds of "mashups". The APIs didn't even require developer keys and didn't have stringent rate limiting. Everyone was giving away their data for free! It's hard to believe now.

Around the time of Reader shutdown, Twitter locked down their API, significantly curbed 3rd party clients. Facebook and Google abandoned XMPP. Later Google Maps became serious about monetizing their API, and all those mashups and RSS feeds started dying down.

So in short, shutdown of Google Reader was the beginning of the end of Web 2.0.

Fast. $0. Straight-forward UI. As mass-market as any RSS Reader got.

Arguably it was The RSS Reader.

Killing Google Reader effectively killed RSS, shifting the gravity of the big tech companies from a fundamentally open & interoperable consumer web to a fundamentally closed silo.