One of the really useful features of RSS feed categories is that they tie into the filtering systems in many clients well. The user feeds have just the title, and tags for categories for questions, and no categories for other post types. There's some metadata, but we could have better metadata.
Here is an example of a question. It has lots of useful metadata for filtering, except an explicit sign that its a question
It would be nice to include the post type in the category/tags (so the example I gave would have "Question, Company, Discussion, Status-Completed" - so the question tag + metadata for what post type it is
It would also be nice to have the full set of tags in an answer (and an answer tag as above) and a comment tag for comments so we can use the native RSS feed filters to help sort through content.