Categories and tags in blogs are useful. They help readers filter the content. Most blogging tools, like WordPress, offer extra feeds based on categories and tags too.
As for WordPress, once the FeedBurner plugin is installed, all your tag and category pages point to the FeedBurner feed, which is the main feed, as alternate link. This behaviour is especially annoying when you try to subscribe only to a single category of a blog. Unless you’re familiar with the url-schema of the blog and know how to point your news reader exactly to the category feed, you’ll end up seeing always the main feed in your reader.
Yahoo! Pipes is very handy at that point and makes mashup and post-process syndicated data very easy. A quick developed Yahoo! Pipe helped me to find a workaround for that issue and add the tumble category to my tumblelog, where I aggregate almost every trace I leave online.
The Yahoo! Pipe is quite simple: enter a blog url and category (or tag), hit run pipe and see the blog feed filtered by the desired category (or tag). It worked for me on a WordPress feed published via FeedBurner, a Serendipity feed but had problems with a Moveable Type feed. I didn’t spend too much time figuring out what’s wrong with the Moveable Type feed, but since it’s published everyone can play around and fix it.