Rojo is the first aggregator I've seen that incorporates the concept of tagging into the feed reading experience. I particularly like that I can now apply multiple tags to my feeds, so that one can read in multiple contexts. There can now be overlap between my KM feeds and my Regular Reads, for example. This is opposed to the current framework of folders where each feed only fits into one folder. This might be enough to get me to jump away from my current aggregator.
The tool also lets me tag individual posts to give you that social bookmarking flavor with a "tags" page that shows the tag cloud of all recently-tagged stories for that
What's missing is the acknowledgment of the "tags" that blog owners have already applied to their posts. Many blogging tools let you add a category, and the Technorati tags tool works on the assumption that the post category is a tag. If Rojo were to incorporate these tags, then I could look at a tag cloud of everything in my own feeds, and I could drill right to the posts in my feeds that are tagged on topics I want to read.