Danny Ayers has come up with a very cool RSS Module proposal:
Simple Semantics Resolution. This
allows RSS 2.0 to support RDF (and all the cool semantic processing that
enables) by simply including a reference to a XSL stylesheet in the RSS. Of
course, then we'd need some end user tools that actually use some of the semantic
features in RDF, but that might be a chicken & egg kind of problem.
I'd love to hear what aggregators are using things like Dublin Core metadata for.
As far as I can see <dc:subject> might be used by
some, but that appears to be the extent of the usage.

Another Module I'm quite interested in is Easy News Topics from
Paolo Valdermarin and Matt Mower.
I need to do some more reading on their concept of a cloud, but I think what it
would allow is automatic blog classification by sites (say… JavaBlogs
using Classifier4J). Then if aggregators supported it it could allow
filtering based on topics, as defined by a particular cloud.