Here's a feature I would totally dig in my newsreader: "Subscribe to this feed for a week".
The use case is this: You see some interesting post somewhere, you probably surfed your way there through 3-4 references. The post is interesting so by association the blog it's on/feed it's in is also mildly interesting. But there's a catch: If you just subscribe to the feed then 9 times out of 10 you're going to have to waste time later pruning it out of your feeds because your interests are only tangential to the feed - you just happened to meet that tangent on that particular day some time ago.
In related missing features. Why doesn't del.icio.us. have "subcribe to the feed advertised in the page where this URL is" - which is sort of the same thing.
This would require some feed metadata contract but I could see that spreading like wildfire - and most blogs specifiy some kind of feed on all post pages anyway.