Google’s feed reader has an option to sort the feeds by relevance. It’s only available if you go to “All items” and select “Sort by auto”. The cryptic name should mean that Google Reader “prioritizes the items based on their importance to you” by analyzing your reading patterns.
This is especially useful if you go on vacation and don’t read your feeds for a week or two. Instead of reading thousands of posts, you can just check the most important ones and then mark all as read.