There are already a bunch of startups. Facebook and FriendFeed are basically both information filters. So are Tipjoy, Reddit, Digg, SU and HN.
Those all let you be selective of what you take in. Filtering what you send out is also useful, but I don't know what sites besides OurDoings help with that.
I disagree. The areas of email, movies, books, ecommerce are wide open. Even news filtering could use improvements.
I didn't say there was no opportunity - just a lot of people trying to solve problems in the space.