I'm really growing to love Google Reader!
The interface is fantastic. The keyboard shortcuts make using it a joy (not to mention, really fast).
Normally one of the main drawbacks of an online RSS Reader is that you can't use it when you are offline (on a train, plane, whatever), however Google Reader overcame this shortcoming by launching Offline functionality in Google Reader. You can now read the last 2,000 posts while not connected, if you need to!
The ability to Star and more importantly Share your posts is an incredible boon. I then use Mario Romero's Reader application to embed my shared items into my Facebook profile.
As I mentioned previously, the mobile version works really well (except on my Nokia E65 the numeric shortcuts don't work) and as it is online, it is always in synch with the desktop version.
What other features of Google Reader are there which I neglected to mention?