Circles sort of make sense from a design point of view, but suck from a UX point of view. They are too time-consuming to maintain for a user. I think adding some sort of tagging system could be a better system, so I could use tags to say "this tweet is 'tech' related" or "'sports' related" and then followers could decide if they only want to follow a subset of my tweets by tag. Would make search/discovery easier too.