l.m. orchard just gets to the tip of the iceberg in discussing potential applications of exposing webfeed aggregator internal data. There's lots of fun things that could be done if aggregators made what you're subscribed to and what you've read available in more programmatic fashions. About the best we've got so far is feedroll export, NetNewsWire's AppleScript interface, and a few aggregators that support plugins, mainly for pulling data in.
If you're worried about privacy, the things I'm thinking about don't even require sharing that data with others. However, I think it would be an interesting research project to make sharing possible, and then design some mechanisms to support the construction of social boundaries.
As an aside, a colleague objected to usage of the term "blogroll" to refer to a list of webfeed subscriptions. Blogrolls allegedly referred to the (poorly maintained) lists of sources bloggers posted on the front pages of their blogs. I think I'll just compromise by using feedroll instead. Same connotation, less confusion. Thanks 0xdecafbad!!