In both cases, one pieces that is currently missing is a feedback. It's quite possible that the people you've shared an item with would like to comment on the item, but alas, currently they have no way to do so (other than "re-share with comment", but that's pretty lame).
All right, suppose they provide this ability. It now links the person that shared the item with the person that commented on the item - but further down, would it be reasonable to propagate the comment to all people the item was originally shared to? What about privacy controls? What about comments and privacy controls on items that were re-shared further? When does the propagation stop, and how far back does it reach?
Non-trivial questions to answer, but I'm sure they will be answered this or that way pretty soon. Mark my words.
UPDATE: As of March 11 2009, Google Reader supports comments on shared items. Less than a month - that's stellar response time :)