David Galbraith: "From a market perspective, surely it would be better if Atom worked around a core of RSS and if need be then the RSS 2.0 core should be amended to include necessary things like the encoding type of content."
The Motivation page on the Atom Wiki is pretty clear about why we need to look beyond RSS. Tweaking RSS 2.0 will buy perhaps a year or so, and then we'll be back hitting the walls again. RSS 1.0 would give a lot more extensibility (and hence shelf life), but doesn't take into account the posting API(s). Time to look forwards.