With OS X Mountain Lion, Apple removed the support of RSS in both Safari and Mail. If you depend on the RSS reader features, look for alternatives before you make the switch to OS X Mountain Lion.
Apple has decided that RSS reader function is best left for third party apps. There are a number of RSS reader apps in the Mac App Store, you might want to evaluate NetNewsWire and Reeder for a start. The following are list of RSS features removed in OS X Mountain Lion:
RSS reader in OS X Lion Mail app is bare bone but usable. This feature is totally removed in the Mail app in OS X Mountain Lion.
OS X Lion Safari comes with a RSS reader supporting the “feed://” URI for displaying RSS feed. This feature is removed in OS X Mountain Lion. The “feed:” style URL is a standard for RSS and Atom feeds, and you need a third party RSS reader in order to display the content.
The RSS button at the end of the address bar is removed from Safari in OS X Mountain Lion. This button enables a quick link to display the RSS feed for the site, making it easier to subscribe to an RSS feed. If you would like the RSS button back, you would have to rely on third party Safari extension such as the Subscribe to Feed extension.
With the removal of RSS feature in Safari for OS X Mountain Lion, bookmarked RSS feeds in Safari Bookmarks bar or Bookmark menu will no longer automatically refreshed.