I keep thinking Instapaper and this service could do more with this valuable "items to read" data. This information has infinite value: they know what I'm going to be like (read) in the next few days (future).
Of course it makes the implementation more complicated, but it could also make the site invaluable. At least, show reviews from other people who read the same links, track "hot" and top rated links (because people just put too much stuff in their reading list and have to filter it anyway, they might as well help each other sorting what is more important).
More complicated things would be: automatically separating links in clusters (categories), telling me some link is no longer valid (some info only has value when new), detecting friends as "people who read (or like) the same stuff, etc.
Right now those sites have little more value than a txt file (or Org-mode), and that makes me frown.
