Well, it's taken a while, but Google Reader's social features are finally taking shape, though their sharing feature is essentially a 'reblog' feature, simply publishing all the items you want to share on a public page - here's mine. The feature would be better off targeting your gmail contacts and allowing you to share an article with a specific person/group (either via email or via the reader) - after all, 'share' and 'republish' are very different things.
The obvious tagging feature is there, but no powerful tag navigation method is on offer and the search facility is ironically non-existent. So there's still a lot of work to do on the navigation front - the most crucial design problem for a newsreading app. I think the navigation itself needs to have a social dimension (tag-driven content drawn from 'pools' of articles, not just your own explicit subscriptions).
Interesting to note the attention data is now pretty much collected real time, and a lot less crudely than in the first generation of browser based RSS readers - as you browse a particular feed, scrolling through the articles makes the feed 'articles read' count go down article by article, meaning that the reader knows if you really have set eyes on an article or not. Google's concerned with capturing accurate attention data - and it's beamed back at you on the reading habits page; right now an interesting set of trends, but pretty useless until the reader starts recommending articles/feeds based on the data. Kind of like our last.fm profile pages - pre-radio, pre-recommendations. So with a pinch of collaborative filtering and more interface nous, Google Reader is set to be quite a big product. As it is, Gmail still seems to be losing the interface wars against Yahoo! mail (i find Gmail incredibly efficient myself), so it'll be interesting to see how the newsreading attention is distributed amongst the big players. It also depends if they can ever recover from their disastrous first impression (the first version of the Reader was truly a shocker and put lots of people off, myself included).
Whilst i'm not using Google Reader (I use Clippr and NetNewsWire), I've become a regular user of google notebook of late. Very quick to use, and comes with a firefox extension. May have something to do with using 4 computers on a daily basis.