How to make Drupal 7 render taxonomy tags as RSS categories?
This used to be the default behaviour in D6, but was accidentally removed from D7 after some refactoring. The latest versions of D7 contain a patch that allowed a third-party module to show RSS categories.

Step 1: download and install the RSS field formatters module.

Step 2: navigate to /admin/structure/types

Step 3: click "manage display" next to the desired content type

Step 4: expand "custom display settings" and make sure "RSS" is checked:



Step 5: select the "RSS" tab (after hitting Save if you had to enable the RSS checkbox). This one is a bit hard to notice:



Step 6: add the Tags to the fields list if not already there. Set the format to "RSS category":



Now save and enjoy your taxonomy tags in your RSS feed!