Just figured this out, the key to raising the EX Gauge I've found is initiating Chase, you seem to gain more than usual as long as you keep up chase chains. Since Sephy loves launching people into Chases, I noticed I was able to activate EX-Mode in most of his fights. Its a shame his EX-Mode isn't that good...

Sephy got a new short range swipe that launches opponents into Chase mode. Beyond that, it seems like they slowed down some of his animations to make it smoother to the eye, it doesn't seem to have affected his speed. Beyond that he's pretty much the same.

Jecht got a new mid-range ground HP attack that is surprisingly faster than I thought it would be. The timing for his EX-Burst seems a bit different as well but I might be imagining things.

I noticed that dodging moves in Chase mode is much harder, the timing for the HP attacks have been altered so I was getting some nasty hits from people like Ultimecia, EX-Death, and Sephiroth.

Also, while Bartz is unplayable he does showcase two new moves from Laguna and Yuna. He has two ground HP attacks, one is short range and summons what looks like Ifrit (its really fast so its hard to tell) who fires a wall of flame in front of Bartz and can be used as a shield. His other attack involves summoning the head of the Ragnarok ship and using a beam blade from it. The attack looks like Squall's Blasting Zone.