Okay after an hour of playing the demo, here's what I've noticed
-The dash in "follow up attacks" are quicker, which is nice IMO
-New stages that weren't shown in the trailers such as the stage in FFIX where Zidane and his bros perform their plays
-Old characters get new attacks and some rebalancing? I think I've seen Cloud using a Brave version of his Cross Slash. Speaking of Cross Slash, I think it has longer range, but this needs further testing due to Cloud not being playable in the trailer and you can only play against him
-Not being able to use your EX attack due to EX Bursting is nice
-This may need some further testing but it looks like if you try to use your assist to Combo Break at all, it fails and you can't use assist for a while.
-Lightning looks like she has some nice shit. Straightforward character, quick Brave attacks, 2 zoning Brave attacks and one of them, the attack where her Gunblade shoots bullets, looks like it can be used as a nice Oki tool.
-Cloud still easy as hell to parry against, stay free lol.