Just played through the demo, Law is ridiculous. If you use R2 to create the room and then combo, it has a vortex effect which even works on bosses. Hit 1000 kills in about five minutes because he can get his full combo on twenty people at the same time. Visually, the rooms everywhere are annoying but turning Enel's head into a bomb more than makes up for it. It's a shame his "shock" move is aerial only, and his transformation is boring, I don't even know what he's doing in his transformed ult.
Luffy has been polished a little bit since the first game. His hits look a little more rubbery, and the sound effects when he hits people are a nice touch. His moveset is actually smaller than the first game thanks to the removal of Rocket and the replacement of his special move wheel with just jet pistol (Or maybe you can unlock other specials?). Gear 2nd is cool, and he uses king's Haki when he transforms. It's a shame they don't update his dodge though, it should be soru! His ults are nice although hitting people with jet storm is a challenge.
Team attacks are activated by transforming, filling a bar by attacking and then pressing circle. They last a fair bit longer than in the first game, and you can move around with the swapped character, so it seems we'll at least be able to play as the support characters for short bursts (I'm guessing they'll only have a couple of combos). In the demo, Luffy has Usopp as his support and Law has Zoro. Usopp I only tried once and his moveset is pretty similar to the first game. Zoro has some cool effects on his attacks now, and he has the tornado as an ulti.
The level itself is much better than what we got in the first game. the hills are a nice touch, and the removal of rooms you have to clear makes it less painful, enemies are in clumps now instead. Also, it seems that they are all on the map from the get-go, which means you can face truly massive crowds of enemies. Also, the stage is a little more dynamic than in earlier games. It's cool when the storm starts, and when you face Enel, it's on Maxim and it starts flying. And Enel's transform turns him into his element, which makes him invincible, so he should be fun to play.