Good questions! Both are actually already supported:
Categories and tags: Both WordPress categories and tags come through. If you look at any story from that feed, you’ll see both — for example “Air, Americas, Technology” (categories) alongside “Arctic, Canada, GPS” (tags). They all show up as story tags in NewsBlur.
Excerpt: NewsBlur pulls the full content.rendered from the WordPress API, which includes everything the excerpt would have and more. There isn’t a separate excerpt field in RSS/Atom feeds (or in NewsBlur’s story model), but the full post content is there. If a post has no content, NewsBlur falls back to using the excerpt automatically.