Watched TB play it, and saw Trump play it as well.
Both after having almost finished a Prince run said that it is probably the most feature complete Civ game at launch. Didn't seem to miss anything from Civ 5 (including expansions).
From what I recall from the Trump part (Also on his youtube channel)
- Combat units stacking makes it more likely to use units later in the game since it's not such a hassle moving them
- Appears (said it is not possible to tell from one game) that there are more ways to win and block said wins
- Involvement of city states are improved (IE: The envoy system and ability to use their armies)
- UI improved. Seemed more detailed, but also cleaner.
- AI still not the smartest, but Trump, looking at the health of a city when attacking it, noticed that there was a "prince" stat boost modifier on it. He also mentioned that the AI seemed to produce less troops. So he is guessing that in Civ 6, other countries will have increased stats as you go higher in difficulties, rather than swarming you with enemies.
Granted only watched a few hours of the game, but didn't notice anything missing.
EDIT: