The ranking algorithm was designed to prioritize the posts from subscriptions you frequently read and the posts that are popular among your friends and other Google Reader users. "Try clicking the like button on things you think are important or enjoy reading, and we'll learn to put items like that first," suggests Google.
Another change is that the list of recommended feeds and the "popular items" feeds have been consolidated in the new "Explore" section. "We use algorithms to find top-rising images, videos and pages from anywhere (not just your subscriptions), collect them in the new Popular items section and order them by what we think you'll like best," explains Google.
Google Reader is now more clever, as it uses attention data to personalize your reading list based on your past behavior.