Marc Hedlund's article for the O'Reilly Network provides a nice discussion of using the new Bloglines web API. Many people are really excited about the feed caching, item management, and distributed synching that the services provides.
What really floats my boat though is the access to the subscription information. With a blogroll management UI outsourced, new services can be built that just look at people's blogrolls. If a significant number of people make their blogrolls public, then new interesting social services can be generated. Share Your OPML 2.0 could be just around the corner.