The full thread text isn't available in the RSS feed. This is more as issue with the RSS implementation by the forum software than RSS itself. It's great at providing notification for new threads (ideal in something like the MR forum), but not so great at seeing the entire thread.

Some RSS readers for Safari would include NewsGator (I think), Google Reader, and Bloglines. NetNewsWire is often regarded as the "best" standalone RSS reader for Mac. Of course, Firefox for Mac is also going to support the RSS extensions...