In Safari 6, part of Mountain Lion and released separately for Lion users, Apple removed RSS support. An RSS button will no longer appear next to a website’s URL when a feed is detected, and Safari can no longer preview feeds.
Daniel Jalkut decided to fix Apple’s unfortunate decision with a “more-or-less unsupported” Subscribe to Feed Safari extension. When you know a site offers a feed, click it to open said feed in your favorite reader.