I'd miss Heroes, Corporations, Barbarians, and Secret Societies, in that order of importance
Heroes probably aren't making a return
If they were, it'd have been as unique Army Commanders, which there doesn't seem to be
(In fact, the seeming loss of Great Men is the mechanical change I most lament, much more so than switching civs)
Corporations could very well be in the modern age, especially since there looks to be an Economic victory
Independent Powers are an improved version of the barbarian Clans mode, so no loss there
Secret Societies are a loss, but not a huge one