From time to time I see people on the internet who remember Google Reader fondly, and miss it. At the time, I didn't have much use for something like it, so I never used it.
But a common theme in these conversations about Google Reader is that no other feed reader ever offered everything Google Reader could, but I can't seem to find details.

So what was it? Does anyone here remember Google Reader, or even still wishes it was available? Why did nothing come from the Open Source community that could replace it?