Sharing items is one of the cool features of Google Reader as it lets your friends know about intersting stuff you are reading. And now you can comment on your friends’s shared items in Google Reader just like Friendfeed. To add a comment on friend’s shared item, click on Add Comment at the bottom of the article.
And if you are looking for only those shared items that have comments, click on comments view under Friends shared items in sidebar. Also, in the friends shared items list, favicons of your friends will have a small comment icon if that particular item has a comment. You can thus read it first as it might be more interesting than others as it has comments.
Comments are available only on shared items and not on All items.