Yeah that's a good question. How much does the 'all posts' page help with this right now? Do you have a sense of how common it is for new users to post interesting things but not get any feedback? HN has an analogous problem with the new page. It's hard to get people to remember to go to the new page and vote up interesting stories, so lots more stories could go on the frontpage than currently do. If everyone voted on new stories I think the frontpage would have a lot more churn.. One idea is to have a 'right hand column' for stuff the user isn't following. I notice facebook and techmeme are using the right side of people's increasingly large monitors to shove stuff their users don't directly ask for (ahem, ads). Perhaps we could do the same but towards a more civic-minded end? Another idea I've been toying with is more radical: topics. I've been chatting with a friend about building a new social aggregator where topics are first class entities just like users, and every topic gets a 'front page' just like every user does. Anybody can connect a story with tags/topics, etc. It's still a half-baked idea, but I want to steal features from Quora and delicious.
http://hubski.com/tag?id=politics A couple of redditors recently tried something somewhat like that with wubel.com http://www.wubel.com/channel/technology We've talked about the right-hand side for discovery, but like you said, it's kind of a force-feed. That's why I am kind of leaning towards users electing to have some portion of their feed be non-followed content. It would be obvious which posts were coming from the outside, and you could toggle how much of it you wanted to see.
Another idea is to highlight active tags. Clicking on a tag to see a mostly empty page discourages further clicking on tags. It might be worth trying out to have random stuff show up on user feeds. Make the background a slightly different shade to distinguish it. One key is to be able to dismiss such stories from the stream. I can tolerate annoying stories if I am able to hide them.
That's my current thinking. If I could see them for what they are, and dial it up or down, then it might be a good approach. Feed feeling stale? Dial up the outside content component. Too much noise? Dial it down.