A volume slider per feed, and perhaps also a global one.
If a feed is posting too much stuff, but I still like some of the stuff on it, I'd prefer not to unsubscribe if I had a better option.
If I notice a feed getting a bit noisy, I'd like to open a view for that feed, with titles and timestamps. Then I'd just move a slider and watch stuff drop out, until I've turned it down to a volume I can handle.
Hopefully, the lower-signal items will drop out of view first and the higher-signal items last. The more data available for making these rankings, the better. Comment counts or scores from the hosting site should help. So should links/shares/likes/comments across the major social networks.
I like the volume slider ideal; I that you could leverage social media outlets as a scoring system. (I'd include HN in that - the more points, the higher the score.) That could give you a nice way to "bump up" the things past some "volume" you haven't seen in priority.
I don't like the idea of "just don't subscribe"; why can't the tool figure this out for me?
Edit: "aren't nice" -> "aren't that nice". I do think counts are useful, just far, far from perfect. What I find myself dong is finding a feed with unread items, skimming, then marking all as read. (Well, when I use Reader, sigh.)
I used to have a ton of unread items sitting in the feed reader; now everything is either immediately discarded or immediately shunted into Pocket. (A good phone interface for me might be: swipe left = discard, swipe right = send to view-later service of choice and discard. Fast!)
Of course, that means I now have a ton of unread stuff sitting in Pocket. So I think services in that category could benefit from this kind of scoring too. I can see my Pocket stuff in reverse chronological order. I'd love a really intelligent, adaptive “sort by magic” for that big bucket.
You suggested using HN as a source. That's absolutely along the lines I'm thinking. It's like this: every site can be a source of stories, but every site should also be a potential source of scoring signals for stories -- either its own or elsewhere.
I for instance, can't be bothered to read all of Hacker News. So I read only the posts that have 150 points or more, via http://hckrnews.com/ (a by-points curator) in the form of an RSS feed.