Often I am only interested in certain categories of posts from a blog. Is there any feed reader that allows this? Also, is this information even included in RSS/ATOM?
I don't know of any reader that has such capacities. Sometimes blogs themselves offer a feed specific to one category.
I personally use Feed Rinse for some feeds to filter out posts that I don't want. There you can block (or allow) items based on title, tag, author or content. Maybe that could help you as well.
category
element, although it's optional. Blogs that have categories are likely to include it. – cjm Aug 6 '11 at 9:09