I'm pretty sure that like me, Jack Vinson uses his aggregator as a knowlege foraging tool, but he's way more demanding than I am. Check out his list of features in an ideal feed aggregator. Then again he earns his bread in knowledge management.
One interesting theme that has been lost through the marginalization of Radio Userland is that space of features oriented around writing from your aggregator. NetNewsWire + MarsEdit is pretty good at this, although the coupling doesn't strike me as tight. I don't know of any other desktop aggregators that attempt this, although I confess I haven't been kicking tires recently.
The low end of aggregators has been commoditized by Yahoo!, MS, Google, et.al., but there's plenty of room for innovation and niche action.