It's mostly about the synchronisation. For my use-case, I read articles in Google Reader during my lunch break at work. When I'm browsing at home later, on my own PC or iPad I don't want to have to wade through dozens of articles I've already read to find the new stuff.

I guess there's also the convenience of access to the web app pretty much anywhere, but that's secondary (to me).

1. local memory consumption. 2. synchronization between multiple computers.

I am testing quiet-rss. I also tested RSSOwl. In fact I used to use Firefox's Brief a long while ago and I used it for extended period of time. None of these comes close to Google Reader for above mentioned issues. GR just works.

Aside from synchronization between multiple devices there's also Google Reader's enormous cache of old articles. The RSS feeds themselves usually keep only a dozen or so of their latest entries while GR usually presents the entire history. Sadly, I don't think any replacement will be able to duplicate that.