"Atom in Depth" Presentation
Sam Ruby gave this presenation at the XML Conference last week, and it details the motivation for wanting to rev another syndication format. It more or less boils down to tightening up the spec so there's less ambiguity in what is allowed where in a feed. The presentation does a great job of comparing and contrasting how the existing specs (and psuedo-specs and spec interpretations) handle the different feed elements.
One other thing I wanted to call attention to is the format of the presentation itself. The slides look beautiful in Mozilla, less so in IE 6.0, but do a View Source on a couple of those slides. Isn't that fabulous? Snaps for Sam Ruby for the awesome and appropriate use of CSS.
Source: Sam Ruby: Atom in Depth