Here's a Wiki which compares the RSS 2.0 and Atom 1.0 feed formats.
The RSS 2.0 specification is copyrighted by Harvard University and is frozen. No significant changes can be made and it is intended that future work be done under a different name; Atom is one example of such work.
The Atom 1.0 specification (in the course of becoming an IETF standards-track RFC) represents the consensus of the Atompub Working Group within the IETF, as reviewed and approved by the IETF community and the Internet Engineering Steering Group. The specification is structured in such a way that the IETF could conceivably issue further versions or revisions of this specification without breaking existing deployments, although there is no commitment, nor currently expressed interest, in doing so.