The Balance improvements, Bug Fixes, New Features and Modes in the Free Updates are the things I enjoyed the most with NFP (Although they don't belong to NFP, we wouldn't have got them without it).
The Civs/Leaders are quite Nice, especially the Concepts of their Abilities, but the implementation of some just make them too OP compared to Previous Civs/Leaders. Overall, they are fine IMO (just need a little balance here and there and everything will be great, nothing that can't be achieved via Modding). I just hope the Update of the 2/3 older Civs/Leaders don't make them similarly OP, otherwise, ALL our Games gonna be won before the Late-Game.
The Game Modes, like someone said here, sound good on Paper, but the implementation of them wasn't that very promising.
Secret Societies was so anticipated from the Fans, but at the end we got all our hopes crushed. We expected anything, but not what we finally got. I like the Concept, but I would have enjoyed it if it wasn't full of Fantasy stuff. Something like Assassins instead of Vampires, and Alchemist that can place a random resource on a Tile rather than Ley Lines. Something more believable would have been enough for me to enjoy it. And same thing for Heros Mode (Generals and Admirals would have been great as Heros IMHO).
Anyway, I think there are many Players who love those Modes, so each his/her.
The other Modes (except Apocalypse) are pretty good, they just need some Bug Fixes (Monopolies, Barbarians) and AI to handle them well (Dramatic Ages is such a good Mode, but as long as AI suffers from it, I can't play with it).
The Good thing about The Modes, is the ability to turn them on and off individually. I would love to see previous Mechanisms/Systems managable like that (World Congress, Loyalty, Individual Disasters...etc).
I'm not whining about all the Bugs introduced with NFP, because that was unavoidable given the Time Frame of the Releases of the Packs and Updates. And I think we will get most of them sooner or later fixed anyway.
If NFP was mostly made to gather some cash for future Civ VI content, then I think we will see more Bug Fixes, Balances, AI and UI improvements with a potential Final Frontier, or ideally, some Free Updates.
Looking forward to the Next Pack and the Mode that it brings (Colonisation? Ideologies? Dynamic CSs? or some alternate Future Mode (Zombies/Aliens - I would have prefered this (Sci-fi) over Fantasy Stuff)?)