Drupal 8 embedded with a plugin system. In drupal 8, Plugin is a group of PHP classes used for a defined specific usage. You can use any registered plugins, add new plugins and also create your own plugin type.
Drupal 8 embedded with a configuration system which allow users to store variables in the database and usable in site wide as variables in drupal 7. Intend of use variable_set variable_get variable_del, in drupal 8 use Configuration API.