Some more info:

From what I heard, there's over 50 summons in this game.
Summons are devided in 3 categories: Flying, Melee and Range.
There are 5 elements (Fire, water, earth, thunder, none).
Summons are also devided in ranks (Like Shiva = Rank III, Darling Shiva = Rank II, Baby Shiva = Rank I)

Mist quickenings are back. Some Mist quickenings and summons will be obtained through sidequests and not through the main story. (Like XII, I guess?) The License Ring also has a hand in how many summons one can call upon at the same time (or something like that, not sure).

The airship you'll be flying around in, the Beiluge, is enormous. In the Trailer Volume 3 one could see Vaan walking around in a Town, right? Wrong. That's not a town, its the lounge area of the Beiluge.

Also, the game's main story will be around 50% of the game itself, the other 50% will be sidequests (Like FF XII, huh?).