- removal of global $action
- removal of Xml::hidden()
- broken Output::add() (had to migrate to resource loader)
- various parser tag bugs
- removal of MessageCache::addMessage()
- removal of ts_makeSortable() (javascript)
- brokage of WikiEditor adaptation
- MediaWiki:common.js no more loading by default (security)
- addHandler() javascript broken in IE8
Documentation seems partially the answer here. $action being available at any point was an accident afaik. Regardless of any precaitions we take, the breakage of $action would probably not be ptevented. (That said most of the other issues mentioned should not have happened).