In Ys Seven, series hero Adol Christin has washed up on some faraway shore -- again -- and is saving the citizens from whatever evil is looming -- again. He's joined by his pal Dogi and a rotating cast of other party members. New to the series is a three-character party, with the ability to easily swap between members at any time. Dogi is large and extra-strong, the other character with me at the time had ranged attacks and Adol is the "average" character between them. In addition, each character has special attacks that can be launched when a meter fills. The AI characters continue to fight with you when you've stopped controlling them directly.
Characters get even more skills as they use their weapons. Each weapon has skills whose levels increment with use. Not only do those skills get more powerful over time, but at a certain point, the character learns them for good, regardless of the weapon in use.
The dungeon XSEED demonstrated for me had an interesting mechanic: it was full of huge fans constantly blowing currents of air around. Normally, this had no effect on my team, but by equipping a special cape, I could be swept up in the wind, which allowed me to travel from platform to platform, removing the cape to return to the ground. However, those pathways between platforms also had dangerous spiked areas, requiring some quick maneuvering. Or, in my case, some quick extreme HP loss followed by healing.
It may seem odd that the first Ys game XSEED is choosing to release is the last, chronologically, but the XSEED rep explained to me that the nature of the series makes it unnecessary to follow the story in "canonical" order. Since the setting is always new and Adol is pretty much a blank slate, the backstory doesn't figure into the gameplay too much. It should, then, be safe to start with Seven and then decide to pick up the subsequent releases of Ys I & II Chronicles and Ys: The Oath in Felghana (which is a significantly altered remake of the weird side-scrolling Ys III: Wanderers from Ys).