For me there are various post types that essentially all boil down to being bookmarks, but the sub-category adds a slightly different shade of intent to it. As an example I've got a "read" post type on my primary site. It takes the bookmark concept ("I want to read this") and adds the additional piece of semantic information that I physically spent some time to actually read it. To an audience, this gives that read-post some additional value over a simple bookmark. Even further, did I bother to make a comment or highlight a piece of material from it? Did I go another step and write a reply to it? Each of these things indicates a higher level of engagement on my part which signals to an audience reading my site, what value I may have placed on a thing.
When it comes to a reader, it would be awesome to have the ability to filter through some of these increasing types within a value chain. People are sharing or bookmarking lots of crap, but what are they spending the time to actually read rather than sharing after only reading a headline? What did they bother to really react to? What motivated them to not only read a piece, but write a 10 paragraph response to? When it comes to finding things in my feeds that are really valuable from friends, family, and colleagues, it's these articles that are usually the most valuable.
You're definitely right that readers are the next big piece. I can't wait for some more competition in the space: http:/