The headline items:

• Complete rework of naval gameplay, introducing the new Hulk-line ship to counter fire ships and relegating fire ships to a more utilitarian function. Also moving all ships to the first page of the dock, by moving some techs out of the dock and into the university.
• Voice chat feature in multiplayer lobbies. Enabled by default.
• Waypoint & gather point flags are unique to each civ now
• Big hotkey changes which might reset some existing hotkeys to default

Some other points I thought were worth mentioning:

• Fixed an issue where units with a ranged charge attack could fire instantly when ungarrisoning from a siege weapon.
• Siege tower base movement speed increase, no longer gains speed with garrisoned infantry, also HP reduction
• Fire lancer cost increase, train time increase

These combined are a great way to nerf the fire lancer + siege tower play that was so bad they had to ban siege towers entirely from the Wololo qualification ladder

• Inca campaign changes to make use of newer units
• Big changes to Inca civ to make use of newer units & buildings
• Spanish missionaries can now carry relics, but have speed reduction while doing so
• Various other civ changes, especially relating to new units
• Added some gorgeous-looking new terrains, including Terrace Cliff for the Incan theme
• New multiplayer servers in Texas and Chile, to improve latency for Central and South American players
• More alleged pathfinding improvements