good ideas here and in the responses. we will consider.
You can show some random unrated stories at the very top of the list, and then stories sorted by collaborative filtering.
In my prototyping it worked super-well.
Another thing is that a lot of value people like me derive from hanging out here are the random things you get to discover here - things that one wouldn't ever think about before and thus never upvote something similar. So it would be good to leave some place to splice in some random submissions that are not related to one's personal preferences.