Plugin system: The plugin systems allows a particular module of subsystem to provide functionality in an extensible, object oriented way.
Caching API: the caching api in Drupal 8 adds cache tags.
The purpose is the ability to accurately target multiple cache items that contain data about the same object, page, etc. For example, nodes can appear both on a page and in a view (stored in different cache items in different bins but both tagged with the same node:nid formatted tag). Drupal.org's Cache tags
