Yesterday night I hit "Mark All as Read" in my aggregator. Nine hours later, I fire it up and bang! 173 unread items for 159 feeds. It's frustrating and obviously not scalable as I diversify my news sources (one might say that I am not scalable).
The current generation of aggregators which rely on manual subscriptions of individual feeds may be suitable for a reasonably stable and modest set of sources, but it's clearly broken for me. And I don't consider myself as a news junky!
Hopefully, a new generation of aggregator is coming, based on collective tagging, ranking and sharing of feeds, plus a bit of collaborating filtering and machine learning for automated recommendations in the more advanced ones. Blogmarks ("Enlarge your bookmarks") and Rojo ("RSS with mojo!") are two examples.
Future generations of aggregators will crawl as many feeds as they can discover and swallow, and suggest feeds and/or links based on your topics of interest -- whether declared or inferred by watching what you and others consider worth reading, tagging, ranking, linking to. Manual subscription to monolithic feeds and TrackBacks will soon look so passé!