Many common functions, shared variables, and constants have been renamed.
Documentation
hook_menu() has been removed from Drupal 8.
Documentation
Files Affected
Automated web tests must be in a PSR-4 namespace, and unit tests must be converted to PHPUnit.
Documentation
Info files must contain a type key.
Documentation
Files Affected
Many common dependencies have moved into core.
Documentation
Files Affected
Static permissions are now defined in MODULE.permissions.yml.
Documentation
Files Affected

This issue can be fixed automatically.

form_set_error() is now a method of FormStateInterface.
Documentation
Files Affected