While I agree that "subject" is a very useful term in the context of syndication...
I recommend adding this to the spec for starters
...it's that "for starters" that bothers me.
We can ennumerate all the elements that appear to be useful at this point in
time, but then we run the risk of having 1. a very long list of elements; 2.
not being able to support something useful; 3. having to modify the spec
every time something else that appears useful comes along.
What about doing subject / categorization as the example "How to extend Atom"