Blocking core issue:
- Turn Rule Chains from tables to configuration entities (http://previousnext.com.au/blog/understanding-drupal-8s-config-entities and http://www.pixelite.co.nz/article/practical-example-converting-drupal-7-module-work-drupal-8). This way Features support will become obsolete and issue will be fixed.
- hook_init() is gone. ThemeKey Redirect could be implemented like described at https://drupal.org/node/2013014. ThemeKey CSS might use https://api.drupal.org/api/drupal/core!modules!system!system.api.php/function/hook_page_build/8. For ThemeKey Debug we need to find another early hook after hook_theme or do it the Symfony way somehow.
- $_GET['q'] is gone: https://drupal.org/node/1659562
- No more variable_get(): https://drupal.org/node/1667896